They might, but I doubt they will. They cast those two well, to get two seasons's worth of them still being children. If they were to recast them, it would probably have been for this season (the Bran actor hit puberty now and apparently changed quite a bit physically between the two seasons, but they kept him). This is one of those things TV audiences are on the whole extremely forgiving about. The changes are distracting for an episode or two, then people get used to the actor looking older than he should, then they get used to it (forgetting the character's age, or blurting the differences out). Personally I would say it's far less damaging for continuity to cheat that way and keep the original actors as long as they're willing to stay then recast them and spark all sort of "the new X sucks, bring back the old one!" or "Arya/Bran are too different" debates. Actors don't like much reprise the role of someone else - it's more difficult to make the role your own, and any change of approach to the part is certain to split opinions a lot in the audience. It can take the audience a full movie or season to get used to a new actor and the new approach to a beloved character, much longer than to forget the awkward growth of an actor between seasons but who manage much better to insure continuity. The rest is all about acting (and writing) - it's an actor's job to pretend to be what he/she really isn't and sell it to the audience.

There's always whole lot of debates over recasting parts in the name of "realism" on the web (for nearly every show or movie franchise with kids in them...), but we rarely see it done, and usually it's because they've lost the actor or there's a too big time gap in a story and the actor is now too young for the part (as happened notably in Rome. The fans massively wanted the beloved future Augustus actor to return for season 2 - claiming he was mature and he could probably pull it off the character in his early 20s, but the production needed an adult (nudity and all that).

There are tricks to downplay the fact a teenage actor has grown up faster than he/she should. The writers avoid hammering the viewers with the timeline in GOT (it's fairly vague for non-readers, unless they stop to think it simply picked up where it left off.. but most who get distracted by this still simply accept the actor as the character), a lot of people won't even question if weeks or months or more have passed for Bran or Arya, and teenagers have odd growth spurts anyway. Bran's puberty will definitely be awkward for an episode or two, but once it's accepted in season 3, the worst will be behind them. There's not much they can do about height or voice (and they probably cast the Bran actor with in mind the built of his older brothers), but a cheat you often see is covering it up in plain sight by breaking briefly the fourth wall (referring to the growth spurt on screen in a scene, then it's out of the way). For the rest, wardrobe/make up help. Keep his face smooth and his haircut childish, cut his costumes to disguise some of the changes of built, and no matter how tall he got and the facial changes, he'll keep a childish side and the contrast with the bearded Jon/Robb actors will do the rest for a few more seasons. Bran can't walk either, so that will be easier not to put emphasis on how tall the actor is getting (avoiding large shots of him on Hodor's back, or using a smaller stand-in for those, is one). Framed properly, it won't show so much he's grown a lot. The worse is behind them: they badly needed Bran to be a child for season 1 and 2. With his arc in season 3, it starts to matter less, and once he's over the Wall even less so. They knew all this when they cast the little guy for season 1, and casting pros are quite used to take all this into account (asking for photos of older siblings to judge what the actor might look like over the seasons and all that. I seriously doubt the Arya actress has older sisters who match Sansa in height, and notice they cast Sansa to get someone who could play very childish for 2 seasons but also would be tall and rather feminine by seasons 3-4).

It's very similar for Arya. They've already cast someone of a certain physical type, who will be able to play "boyish" for many years yet - a good bra and costumes slightly too large for her - they make you look lost in them and smaller - will do the rest. She'll grow, of course (and in street clothes and without her wig and costume she's not exactly boyish) but the strong contrast with their tall and feminine Sansa actress, and the Daenerys actress, will remain. And again, the worst is soon to be behind them, once Arya has left Westeros and be on her own in a new arc for a while.

There's also the "mom's heart" syndrome that makes a lot of the audience simply enjoy to watch the kids grow up on screen year after year, becoming very attached to the young actors over time, and which makes producers always hesitate a lot to recast those parts in the name of "realism" (remember the rumors of recast for Harry Potter. Whatever truth there was to that - probably not much if you ask me - it made a huge fuss among the movies's fans and the producers soon came out of the woods to confirm their intent to keep the cast for all the movies. All of them, main cast and secondary cast, were way too old by the last movie, but the audience just went along with the illusion.

A few recast along the way are to be expected. It's far more difficult in the UK than in the US where actors accept more to be confined to a single role for 5+ years, in fact often wish to land that kind of long-term TV jobs. UK actors (I should almost say non-American actors, it's less a UK thing than the reverse is an American TV thing) are known for hesitating to sign contracts that don't let them re-consider their involvement with a project after 3 seasons/years, as many wish not to be stuck too long in roles. It doesn't mean most won't renew for more seasons, and many will see all the seasons's through (it's not that no actor will accept to go on for 6, 8, 10 seasons), but it wouldn't be surprising some go. UK writers are used to take this into account (lots of deaths, departures and such in seasons 3 of british series... Hustle lost its main lead after three seasons, Spooks had to write off its three main lead in season 3 (two asked to be written off the show during season 3, the third agreed to stay but later decided it would be better for drama if they killed his character in the finale and suggested it to the writers), and again its new lead actor in season 6, and several other main players along the way (later on it was mostly due to the writers, who grew to love that on that show even the lead character could die, and the middle of any episode of any season, at that.). Some actors asked for a break to do other projects then returned a few seasons later, others just guest in later seasons etc. Downtown Abbey also lost 3 main actors. They had to kill their male lead (which apparently caused a huge commotion with the US audience not very used to this), one of the sisters, and to prepare the off-screen exit between s3-s4 of a main servant characrer, Miss O'Brien. US shows that cast some Europeans also had to deal with that - most notoriously with Lena Olin in Alias), but with GoT if any of this happens, it will mean some recast (hopefully not Peter Dinklage or the actress playing Cercei or Petyr or one of the Starks, but with a non-US cast it could well happen after the season 3 mark is hit (it wouldn't suprise me of Aiden Gillen, he's often had one, two, three seasons roles in his career, at least one of which ended after he refused an extra season), Thankfully for GOT, it just happens that many characters have just three seasons to cover... Cath turns into a rare guest star after the Red Wedding (and make-up helping could more easily than others be recast, anyway), Robb and most actors in his story arc that have been around since s1 will go this year, Tywin will die after three seasons (for him), Lysa is just a guest star but she will soon be gone too etc.). Anyway, I doubt they'll wish to recast any part they aren't forced to recast by actors refusing to return - the show is already considered challenging for a TV audience for the number of parts such as it is, and it all works largely because all the cast manage to make their parts, big or small, quite unique and memorable.

I'd hate to lose the current Arya or Bran, personally. I much prefer to retain what they bring to their roles and to overlook the fact they age too fast.