Wullu Wullu:
Zero sympathy for Darwin candidates like this.
Contracts. Read em!
$1:
Phones like this should have a pre-set limit then when you reach that limit it shuts down thus preventing run up charges. I've heard of parents getting limits for their kids phones and I'd want that just in case. If you reach your limit you can call up and increase it but you'll know where you stand.
Nnnooooo.... you should have enough common sense not to run it up to a higher amount than you can afford using your very own brain.
If your child is not responsible enough to do the same, they should not have a cell phone.
Simple. It's called common sense, and accountability for ones own actions.