good luck with finding what you are looking for...however, just a quick opinion...my son is now 12 and in the 6th grade at Joshua...his bd is aug 30 so he is one of the late babies..my older daughter was in private school at the time my son was due to start kindergarten. We went ahead and started him even though he was the youngest. He had great social skills and I just knew he would be fine. He did OK in kindergarten but the teachers advised maybe holding him back as he was still struggling with the work..socially he was fine. However, we didn't hold back and sent him on to 1st..it was a battle with school work every day...the next year my daughter, who was in high school, decided she wanted to go to public schools so we transferred both kids to Joshua (and love it) my son started 2nd grade doing some work that he had already done in 1st grade in private school. I thought this would be a great opportunity to get him caught up academically but it wasn't. He hated school, I hated him and homework every night was a battle. We decided to hold him back in 2nd grade and he has done great every since. The maturity finally caught up with him on the academic level.
Just don't push too hard because it is so different than when I was in school. They learn so much more so much earlier. It can be a real struggle for the younger ones, especially little boys..
good luck..