Here, it seems that diseases and injuries and treatments of those are on topic, regardless of age. Also on topic are Environmental or nutritional factors that affect health" and "The health effects of processes such as pregnancy and aging."
At Parenting.SE, what's on topic includes "safety considerations, food and feeding, health and hygiene, development and growth."
This is a question that directly prompted the meta question being raised. Where does it fit better? Are there guidelines or decision rules about what goes where?
There's also this discussion about pediatrics & children tags.