Um, yeah. I don't think it would stop these pervs. Then again, getting the kids in good physical shape (lots of muscles) probably would help...
There are a ton of books on kid self-defense out there, the best one is probably The Gift of Fear. Very basic stuff -- how to windmill, who to run to in an emgergency etc. etc.
For the record, living in the big city -- I've never once heard one of my friends who are parents relate a story of their kids being approached.