I came across this recently too.

I think it comes from the idea that you should definitely not give skim milk to under two's but over two it is ok, but not necessary iykwim. I found it an odd suggest too.

My thinking is that my very energetic, bouncing 3 year old needs full cream milk. She doesn't have loads of other fat, and doesn't drink LOTS of milk so i think it is good for her to have full fat. I don't think (personal opinion coming up) that it is healthy for toddlers to be on diets that include skim this and diet that. I think a balanced diet with all the food groups is much more important.