Please show me where in the Constitution the federal government is given the power to address such things such as education or children walking to school. I seem to remember that if it isn't listed there, those powers ARE RESERVED BY THE STATES.
A closer reading of the Act (see pages 857-858) says that the parents can't be held criminally or civilly liable if they let their kids - with permission - get to school in an "age appropriate manner." It also says that the Act itself doesn't supersede any local laws.
So: if a state or locality has decided or later decides to specifically make a law specifically against "unattended kids going to school," that law would take precedence over this act. "States rights" are still in effect here.