You are mistaken on many points.
First, these radios require an amateur radio license to transmit with. It is illegal to transmit with them on FMRS or GMRS frequencies as they are not FCC approved for that use on any power setting or frequency.
It is legal to listen with them on any frequency.
It is legal to sell or buy them in the USA.
There is an exception if life or property is threatened, any radio on any frequency can be used to call for help under those circumstances.
Using this radio on FMRS frequency to keep tabs on your kids is illegal. You probably won't get caught, but if you do the fines start at $1500 and go up from there.
As a licensed 'ham', if I make a radio from razor blades and welding rods I am legally able to use it on any frequency in my license range.
I recommend you get yourself an FMRS radio at Walmart to keep tabs on your son. It's way cheaper than getting caught doing what your doing.