It is my responsibility to take care of my child and the responsibility of another parent to take care of their child. The parent of the other child should have gave their child a little talk so that they might not get lost or give them a cell phone or money to make a call.
This is a very hard question to answer and there is no clear right or wrong here. Personally, I would have to say someone else's child. Of course losing your own child is bad, but in that case, someone hasn't entrusted you with their child. I personally think it would be worse (morally) to lose someone else's child, because those parents have got a lot of trust and faith in you for them to place the safety of their child into your hands. You are looking after something very precious and important to them, and losing it, again in my opinion, is worse than losing your own child.
Really there is no right answer. But I suppose the latter, as in the former you will simply blame yourself, while in the latter you blame yourself, and the other kid's parents blame you. Maybe even come after you. Again there is no right answer, but most people would say the first one since "It's my kid! Of course it's worse!" But isn't that kind of a selfish answer?