I spent three years in the US - there is about six months difference in terms of schooling. So when we went there, when my daughter had just finished year one, she ended up going into first grade, repeating about half of year one, and when we came back to the UK, she went straight from third grade to year 5, and was about six months behind. We were concerned, but she caught up very quickly, with no extra help or tutoring.

For younger children, they pick up reading and writing at their own pace anyway so I'm sure your younger one will adapt quickly. Going from a small class to a large one is similar to the transition from pre-school to reception in any case.

I started my son at school early in the US (which we had the option to do at their elementary school) because I new we would be coming back to the UK, and I regretted it, because he was only four in a class of five year-olds. The age difference was much harder to deal with than differences in what they had been taught in ther pre-schools.

Now the only difference for my kids is in their accents - we've been back four years and they still sound American.