If a child is < 6 months old, not much, and don't worry.
If a child is > 6 months old, make sure the food they're eating is healthy. This means no juices or "junk" food. Also, the earlier a child is introduced to "solid" foods, the more likely he/she is to be overweight
. To avoid this, wait until at least 4-6 months old to introduce (soft) cereals and fruits/veggies.