You can certainly try but usually they will be put in the grade they qualify for based on age. They are moving to a new country and a new system. If some have to repeat a grade it is not because they have failed their grade they are entering a new school system where the curriculum may be different and you want them to integrate as best as possible. I would not want my child to be the youngest in their class. There are bigger chances or bullying, not finding friends if classmates are much older, etc.