But what if getting sick with various things as a kid and surviving those things is an important part of human development?

this is a very interesting issue and somewhat nuanced.

some kinds of sick are not good. avoiding measles, smallpox, etc is likely a big net benefit.

but not being exposed to lots of colds, flus, bacteria, eating dirt, etc seems linked to weak immune function and the development of auto-immune issues.

if you are not familiar with it, i suggest reading the link on hygiene hypothesis. i suspect you'll find it thought provoking.