It might be fun for the kid to be the teaching assistant, but I'm not sure it's good for the kid in many ways. Not just how the child is accepted/not accepted socially by others--although I don't think that should be easily dismissed--but in terms of learning time (as you mention Cathy), perfectionism, and self-concept, to name the ones that pop into my head right away.
If the child is the "second teacher" in the class, what does this teach the child about herself? How does he see himself fitting into society?
I was the TA when I was in school, too, but I don't think it was very good for me. It's better than sheer boredom, I guess, but not by much. And I think it causes just about as many problems as it solves.
FWIW...
