I was interested in this too and googled "human proportions" to get a better idea of what I should shoot for. I can't remember the numbers exactly. Plus the eyes of an infant and of an adult are almost the same time, even thought body length is different. Some head / body ratios:

Infant 1:4
Toddler 1:5
Teen 1:7
Adult 1:8

These are not absolutes and some people would say that adults are 1:9 but I don't see that in day to day life.  Also younger people have a higher percentage of body fat which rounds out the facial features.