Re: where to put brake? Front or back?
If you're really worried about the coaster brake failing (less likely since you are aware of it and will be looking at the pedal chain, checking the brake arm bolt, etc.) why not put bmx caliper brakes front and rear, keep the coaster brake and get a double pull lever adjusted so that most of the caliper braking up front, but still some from the back? No, I don't know of anybody with three brakes, either, so be the first.
Someday when I grow up I will probably lose interest in toys with wheels, but until then...