Ok, probably should have done this before posting a thread, but I just googled full cream milk and children and came across a lot of recommendations that you switch over to reduced fat at 2 for the reasons below (sorry I'm not too sure how to do quotes so have just copied and pasted). I still not 100% sure that this is the best thing to do, but clearly the basis of the recommendations have nothing to do with preventing weight gain.
Since 2002, it has been recommended in the National Dietary Guidelines for Children and Adolescents that all children in Australia over the age of two years consume reduced-fat milk. The reason for this is that the type of fat found in milk is largely saturated fat — the fat that accumulates in the arteries over time increasing the risk of heart disease. It is reasoned that as Australian children get plenty of fat in their diets from other sources they do not need extra saturated fat from their milk. A common parental concern about choosing low and reduced fat milks over full cream varieties is that kids may be missing out on calcium. This is not the case, reduced- and low-fat milks generally have more calcium than full-cream varieties.
For this reason, once your child reaches the age of two, a reduced-fat milk is the best milk choice. The other potential benefit of swapping a child's milk over when they are young is that they become used to it before knowing any different and there is no problem swapping it over. We all know how difficult it is to change kids' food once they reach school age!

