We evaluated the LJ Elementary seminar program and decided that although our son tested in, he was better off in his private school, which he attended K-6. He got a lot of personalized attention and ended up way ahead of the standard curriculum. We also did a lot of supplementing outside of school in lots of different areas. Now, after one year at Carmel Valley Middle, he is in SUCH a good place. There are tons of other kids who are math/science nerds, so he has a huge group of peers/friends. The extracurricular math/science programs have been outstanding, and he is so motivated to succeed in all of the math contests. I am so happy with the choices we made, and I think CVMS is an excellent place for this type of kid.
I agree with flu, after-school chinese school (or the equivalent) is the way to go. These kids need to stay with their age group at school, for social reasons. Yes they will be bored and it will be easy. But if they have the extracurricular challenging work, they will be happy and successful.