But (hypocritically?) we got the best of both worlds: single subject acceleration in every subject, without whole grade acceleration
I would throw in a caution about this though. You will likely run into problems down the road if your child's base grade level and subject levels are too far apart. While not an issue in lower grades,it can become a problem at the high school and above level.
At the high school level, some schools won't give high school credit for classes taken before high school (9th). For example, that could mean that you child may have finished all the high school math classes available prior to actually being in 9th. If he/she needs 4 math credits to graduate high school then you will have to turn to college classes. But then you could run into a problem in that a lot of colleges have minimum age or grade levels for enrollment and your child might not yet be eligible. None of these problems are insurmountable but they are obstacles that you will have to work around.
Also, there comes a point of diminishing returns so to speak. Your child might reach the
point where he/she is taking all college courses. At some point you might want those courses (and the money spent to accumulate them) to go toward an actual bachelor's degree rather that a high school diploma. And what happens if your child completes a bachelors degree requirements but has not yet graduated high school? He/she might have to start his/her undergrad taking graduate classes. The kind of college you might want to attend for undergrad might not be the one you want to attend for grad school. You might dream of going to Caltech as an undergrad, but if you plan to pursue a PhD in pure math with an emphasis in topology that might not be your first choice if you are taking classes at the graduate level.
Again, all of these obstacles can be overcome with creativity, but just keep in mind that what works for 2nd grade might not carry through to college.
