I couldn't find anything that might explain why River wouldn't know her younger self. Most of her explanations about herself come from Forest of the Dead, actually, but I couldn't find anything there either.

She mentioned that she knows the Doctor from his own personal future. She talks about spoilers and the Doctor's rule that he can't read the TARDIS-shaped diary. She mentions a picnic in Asgard. She says that she knows who Donna Noble is. She reveals that the Doctor gave her a screwdriver. She says that the Doctor's eyes look younger than she's ever seen. She says that she would trust him with her life, with everything.

What I found most interesting was a quote from the Doctor. He says, talking about plots, "You need a good death. Without death, there'd only be comedies. Dying gives us size."

Of course, it could be as simple as the thing about spoilers. The Doctor isn't allowed to know things that happen in his future, so River never says anything about herself being the little girl, and plays along with everyone else who doesn't know. After all, when she shot and missed the astronaut, she says, "Of course." Perhaps because she knows she wouldn't be able to shoot her younger self?

I'm curious how River knew that it was the right time to appear and tell the Doctor who she really was. If the Doctor jets off and meets her younger self right afterward, perhaps her older self remembers that he learned about her at this point in his own timeline?