Although the Red Cross age restriction is something to refer to - that is not the law - that is only a non-profit company making an educated statement.
The State of Wisconsin child abuse and neglect reporting laws do not specify the age at which a child can be left home alone - therefore unattended at facilities like yours. No consistent community standards exist describing when and under what circumstances children can be left alone or in the care of other children. Information about the age at which it is considered appropriate to leave a child alone may exist elsewhere in your local, county or State policies or ordinances that cover this special topic.
You may want to contact your local police department or child protective services agency for information about specific local regulations or ordinances in your community if there is any.
If you need help in locating your local child protective services agency, you may call Childhelp USA® 24 hours a day, seven days a week at its toll-free number, 1.800.4-A CHILD® (1.800.422.4453).
Additionally, in the State of Wisconsin there is no minimum age requirement to baby sit and no standard for certification under the Wisconsin babysitting laws. However, many teens chose to take a babysitting class - ie.American Red Cross and the Girl Scouts of America.
So, it's a matter of reaonableness under the circumstances - there are no mandatory age requirements for anything in Wisconsin.
I believe that Red Cross doesn't allow children under the age of 12 or 13 to take the babysitting class. Presumptively then (although that would not alleviate your legal liability) the age the American Red Cross allows children to take the babysitting class would be a very strong argument that you acted reasonably when enacting or stating minimum age requirements to use the facilities without parental or guardian supervision.
If they can babysit others they should (although not necessarily so) be able to control themselves and make mature decisions while being unattended.