G.B.
Here is a link to your state regulations and requirements. It appears you can watch 4 children up to age 7 in your home with no license.
"Under Wisconsin law, no person may provide care and supervision for 4 or more children under the age of 7 for less than 24 hours a day unless that person obtains a license to operate a child care center from the Department."
I would think if you want to do this for the actual money you might want to go ahead and do it right with a license and the ability to accept kids that get money from the state to pay for child care. That way you are sure to get that money each month. Parents can be wishy washy and not pay but the state always does if the right procedure was followed for billing. It is comforting to know that money can be counted on. If they do direct deposit just remember if the bank is closed for a holiday the state is too, deposit's are at least a day late when they're closed for holidays like M L King day or Presidents Day....
http://dcf.wisconsin.gov/childcare/licensed/Index.HTM
Here is a link to the actual licensing regulations handbook. It will give you detailed information about what you must have on hand, amount of space per child you'll have to have available, what to have posted in a parent area, all kinds of information.....
Good luck, child care is a wonderful field to be in. Kids need positive role models and it takes all kinds of people in their village.