My MIL actually worked for the state certifying daycare facilities. There are many rules and regs that depend on the state, and the city you live in. Basically if you have some kids at your home that you are not taking care of full time, the rules are considerably less restrictive than if you have children in a home (or building or room) in which you do not live at. If it is the latter, the manual is about the size of a large phone book what you do need to comply.
It can be a great way to earn money if your wife likes kids and you have the set up to do so.