I don't want to kill children - ever. That said, there should be more options to deal with the tiefling kids, but maybe there will be in full release.
I agree, that they probably had no other choice than to harden up and become thiefs, with the exception of Arabella, who has parents that care. But still, you should at least be able to adress it in some (non-lethal) way.

"We are all stories in the end. Just make it a good one."

Doctor Who