Federal, MN State or WI State are you interested in knowing. They are different.
WI law does not require a toddler (or anyone) to wear a pfd.
MN law requires life jacket to be worn by children less than 10 years of age when aboard watercraft in Minnesota when the craft is under way (not tied up at a dock or permanent mooring).
Federal Law (Mississippi, St Croix and such)
In states where no children’s life jacket law is in place, a U.S. Coast Guard interim rule requires children under 13 on moving boats to wear a U.S. Coast Guard approved life jacket that fits.
This rule does not change or supersede the existing law in states which have life jacket laws, it only applies to states that have no laws. Please see the list below for the latest laws in your state.