Bah! You're talking about a few more pounds on a bike for goodness sake! They're not riding in the Tour de France.
Heavier bike = stronger leg muscles
And, as far as I can tell, even the low-end Trek, Specialized, etc. bikes (which cost well more than the mass-market bikes) are heavy and you don't see much weight reduction until you start spending way more $$. (Please feel free to prove me wrong on this point--I'd love to find a low-cost lighter bike for my kids, too.)