Thanks for all the responses. It's good to hear that it CAN work well to have siblings together like this. I agree that it's inappropriate to automatically rule out accelerating a younger sibling. Sibling relationships differ greatly. My daughter actually seemed excited about getting to spend more time with her brother at school, and kind of excited/proud that "he must be a very talented Kindergartener" (as she put it - all I said was that he already knew the math they were teaching in K). I *do* think, however, that it's important to consider carefully the well-being of both children in making a decision like this. If my kids didn't have such a positive and supportive relationship, I'd be much more leery.
However, one factor that has occurred to me since I posted is that my daughter could be especially threatened if my son actually does surpass her in math, because he has already surpassed her in reading (she has had some difficulty learning to read, while he taught himself at 3). She knows that, and she handles it OK, but she thinks of math as her strong suit. I imagine it could be very difficult for her if she ends up behind little brother in both. On the other hand, maybe they will push each other a bit in a positive way.
Oh well. With any luck, I'm overestimating him and he'll end up placed in first! That would make things easier and also pave the way for a full grade skip next year, since he'd have a chance to get to know his new classmates.
