My first thought was that you might be violating Michigan's seat belt law. However, the law actually appears to be fairly lax. For example, one section says:
(5) Except as otherwise provided in subsection (3)(b), each operator of a motor vehicle transporting a child 4 years of age or older but less than 16 years of age in a motor vehicle shall secure the child in a properly adjusted and fastened safety belt and seated as required under this section. If the motor vehicle is transporting more children than there are safety belts available for use, all safety belts available in the motor vehicle are being utilized in compliance with this section, and the operator and all front seat passengers comply with subsection (3), the operator of a motor vehicle transporting a child 8 years of age or older but less than 16 years of age for which there is not an available safety belt is in compliance with this subsection if that child is seated in other than the front seat of the motor vehicle. However, if that motor vehicle is a pickup truck without an extended cab or jump seats, and all safety belts in the front seat are being used, the operator may transport the child in the front seat without a safety belt.
So, transporting a child under 16 in an open truck bed, even at less than 15 MPH, would appear to violate Michigan's seat belt law - UNLESS all seat belts in the truck are being used and the child isn't so young as to need a booster seat, in which case it appears to be OK. (At least under these laws - it's possible some other law covers this, but if that's the case I couldn't find it.)