CRF50 or TTR50 are both good, you can't go wrong with either. I wouldn't bother with a used PW50...I doubt they would be able to pick up a PW50 either...so it's mute point. They will all get dropped at some point...that's a guarantee. Not even worth worrying about that.
Don't overpay for a used one, and when they're done with it (if they don't destroy it) you should be able to sell it for what you paid for it. Then move into the 110's
7 year old boy is on a TT-R110, he is super tall for his age though and he can touch no problem. Last year, he rode it but I had to hold him up while he took off and he would ride it back to me so I could hold him up while he stopped. Luckily he has really good throttle, brake, and slow speed control. He rarely dropped it. He rode a balance bike from 1-3 and was a beast on it. Took it off jumps and shit. He never rode a pedal bike with training wheels. He went straight from balance bike to pedal bike and was riding in 5 minutes once he figured out the pedal situation. He rides dirtbikes like he was born for it. This kid rips trails on this thing.
My 11 year old rides it too, but he's too big for it. I'm looking for a KLX 140 L for him. He's big for his age too though...I think a 110 is good for 'most' 9 through 12 year olds probably. 50s are good for 4-8 usually.