Yes and no.

Martin reputedly lets a lot his series grow organically - leaving himself plenty of room to go where his characters lead him. He doesn't outline ahead in great detail the way a lot of Fantasy writers prefer to do. He knows the great lines of where it's going, has plans for some characters that may or not pan out when he gets there, but there's still a lot of the details up in the air. That explains in part why it takes him so long to write the books. That's fairly close to Tolkien's writing process, and quite unlike the process of people like Jordan, who after he devised the basic ideas/concepts crafted his finale then worked backward to outline most of the great turning points of the series, all the way to the beginning. Then Jordan played connect the dots as he wrote the novels, figuring out how he'd get from point A to point B to C... all the way to the end.

In an interview, Martin has given a few examples of decisions the series's writers made that he told them would likely force them to move away from the books as he had plans for some characters. One of those examples was making the singer who lost his tongue Marillion, or the death of that Dothraki lately. Even in that light, the writers maintained their decisions. There are constraints they work under which GRRM doesn't have, like the actor playing the Dothraki wanting out, and in the case of the singer, because of the medium they most likely didn't care much he would return many seasons down the line and need his tongue. The TV audience will have totally forgotten who that minor character was by then, they have no way to be sure the actor will be available to return either. It will be far simpler to introduce a new singer character then, if necessary. In the meantime, re using the character to cut his tongue out avoided casting someone else, and might have a bit more impact to those who remembered who he was (which wasn't my case, I didn't recognize the actor from last season).

Introducing the Reeds would have required screen time they did not have (to establish their characters, but also their peculiar ways/culture - the plot won't involve that people for some time, if they are introduced in the series at all), and two more young actors. For the time being, they just don't need them, they're pretty tight for developping the Winterfell events such as it is, and early on these two merely hang around.

Maybe they will introduce them later - as refugees they'd bump into or what not (next season? They don't appear to be cast for this year), either to provide more matter for Brandon's story arc or during an episode when they can devote screen time to their meeting with Bran/establishing a relationship, maybe they'll decide to do without them completely. If they excised them from this season, it's very likely their final decision wasn't even made last year, they'll figure out if they really should include them when they plan and write season 3. In the meantime, they spare themselves two new young actors on set (they're pretty restricted with working conditions/schedules/schooling for minors under UK laws, far more so than Hollywood is. Young actors in UK productions slow things down a lot - scenes they'd have time to shoot with adults in one day they'd be forced to split over two days of filming because of the maximum hours they're allowed to let a young actor work each day, and this rapidly becomes fairly expensive and complicates shooting schedules. That's most likely why they also keep Rickon's appearances to what strictly necessary for story progression - they barely ever use him in background scenes, when the presence of the young actor on set when he'd be little more than "set dressing" would complicate their working schedule for very little gain).