Before addressing the question directly I'd like to take a sarcastic aside which I will leave you to work out the significance of...
Why do people buy guns? There's only one thing guns are made for and that's to kill people! These people who buy guns are ready and willing to kill people. They are all would-be murderers. They should have their guns taken off them and be locked up and not released until they have had counseling and agreed not to ever own a gun again! [/sarcasm]
"So who is paying for Child Model site memberships and why?"
There are many possible reasons why people may join these sites... The only reason in common is that the viewer likes to view what is presented on the site. Many people in society are lonely and have no beauty around them. We are not all lucky enough to have a partner and children of our own. Children bring joy to the lives of many people who they have contact with. Not everyone recognizes that but the people who join these sites do. Much like how some people idolize the stars of music and film, others idolize the beauty and charm that children bring. To have a safe way to bring this happiness to many is a very desirable thing, IMO, not something to be condemned.