Given the society she is in, I would say she is in a good situation. She is receiving a certain amount of free schooling while at the same time earning a locally decent wage working for a family who treats her well.
The lack of better and longer free schooling is not Derf's fault. It's the responsibility of the Indonesian government to ensure that all children have access to a proper basic education. The fact that children have to work to support their families is not Derf's fault. Again, that's the responsibility of the government. Derf's not employing iher s not going to make that situation any better. It will only make the girl's situation worse.
No, I don't think it's morally wrong. I think Derf is asking us for approval because he probably does feel a degree of discomfort about the situation, but that discomfort comes, I imagine, from trying to apply our values and mores to a very different set of cultural, social and economic circumstances. But he has to operate within those circumstances, and do the best he can within them. What's morally wrong, in my opinion, is the arrogance with which we Westerners so often believe that ours are the only values which count, and that every situation has to be measured by our standards.