I just came across this thread and thought I'd add what I know, even though it's been a while since you've asked...
My husband moved from the UK to Oklahoma when he was in 6th grade, and he was FAR ahead of his classmates in terms of where he was in all subjects. He was beyond bored with the material, because he'd learned it years before. He told me he felt dumbed down until he started an international school in Houston in 9th grade. The only subject he wasn't up to date with, of course, was US history. It's common for UK schools to teach things earlier than US schools. They begin school at a younger age and they have a longer school year in the UK.
I know people who have gone from the US to UK and the UK to US, and the US will almost always accept school credit from the UK (of course except for Texas history, as another poster has mentioned), but the same can't be said for the UK accepting US credit. So, you shouldn't have any issue when/if they return to the US. It would be the other way around that could be an issue when you first move to the UK.