Re: A bicycle engine kit for a 11 year old? YES or NO
I'll second what's already been written. He'll learn a whole lot especially if you help him motorize his bike and he doesn't help you. You know what I mean, even if in the end you do most of the work, ownership is a big deal. Also a responsibility as the owner/rider to be checking things, riding safely or getting hurt. What could be better than two buddies building and riding together? Father and son. Good for both of you. He'll only be a boy for a little while. What would it have meant to you if your dad had taken the time to do this with you? I'm a little envious as my kids are all grown up. Good luck and safe riding to you both.
Someday when I grow up I will probably lose interest in toys with wheels, but until then...