I've got a Haro cruiser (sold my wife's Felt 2 weeks ago) and also ride mountain bikes. I find that because of the geometry, it's really hard to bunny hop the cruiser, compared to a mountain bike. I'm not a big fan of air time, and find the cruiser feels a bit squirrely when jumping, but that's also a function of the steep geometry.

As far as your kid's bike, tough call. Teach him to be smooth, and the light bike will last a while. Or buy him the heavier bike, and let him wrestle with it a bit. Depends on the kid, really.