[MamaChinch]

My friend moved to the US from the UK when her sone was six and he had to start in K here. She explained it because of the age for starting school here and that systems were different and that he would be like the other kids here. His b-day is in Oct.
He was, although, aware that it was K and not first grade to start off with, but once school started and he made new friends, it was all a wash.
I think if you keep it simple, and you make it a non-issue, they will take their clue from you, espcecially with eveything else going on in your lives!
Good luck, I am sure it will be fine! (probably a bigger deal if they were girls! he he!)