Myself also born in Dec and my mother arranged me to study P1 in one of the Yau Yat Chuen primary school (before that, I had studied P1 in 德貞 but mother wanted me to change to another better primary school). I was a big kid in that year and it's no problem!! I don't have much impact.
So in your friend case, I prefer to study in KV and let him get a better kinder for study one more year.