Is that legal?
Nope.
From the US Dept of Labor website:
The Fair Labor Standards Act (FLSA) sets 14 as the minimum age for most non-agricultural work. However, at any age, youth may deliver newspapers; perform in radio, television, movie, or theatrical productions; work in businesses owned by their parents (except in mining, manufacturing or hazardous jobs); and perform babysitting or perform minor chores around a private home. Also, at any age, youth may be employed as homeworkers to gather evergreens and make evergreen wreaths.