this is from the food standards agency:
Should children have full-fat or semi-skimmed milk?
Milk is an important part of a child's diet. It's a good source of energy and protein, and contains a wide range of vitamins and minerals. Milk and milk products are rich sources of calcium, which growing children and young people need to build healthy bones and teeth.
At about one year old, when a baby has been weaned, you can introduce full-fat cows' milk. If you are breastfeeding you can continue to breastfeed.
Children under two years old need full-fat milk because it gives them the extra calories and vitamin A that a younger child needs.
Children between the ages of one and three need to consume an average of 350mg of calcium a day. About 300ml milk (three?fifths of a pint) would provide this.
Semi-skimmed, 1% and skimmed milk contain at least the same amount of protein, B vitamins, calcium, magnesium, phosphorus, potassium and zinc as full-fat milk, but less fat.
Once a child is two years old, he or she can start drinking semi-skimmed milk as long as they are eating a varied and balanced diet and are growing well. However, skimmed milk and 1% fat milk aren't suitable as a main drink for children under five years old, because they don't contain enough vitamin A and skimmed milk is too low in calories, which children need.
It's important to note that although milk provides a range of nutrients it doesn't contain a significant amount of iron or vitamin C, both of which are also very important for growing children. It's important they eat a varied diet to make sure they get all the vitamins and minerals they need