For a childrens bike, you can likely figure out a way to seize the freewheeling mechanism in the rear hub of an off-the-rack SS kids bike. Personally, I'd weld it in place, but pumping it full of epoxy or something should work similarly well if you don't have access to welding equipment. Since a child weighs a lot less, travels at lower speed, and likely won't be doing any skid stopping this solution should hold up just fine.