But now, here is the paradox. Yes, if the croc is being honest or not is important, but how about this: she could say "you will defenitely not give me back my child." If he keeps the child, he will be lying. If he gives back the child, he will be lying. The only logical thing for him to do with the child would be something completely different, like dump him in the river. She can then fish him out because the crocodile can no longer touch the boy, unless there are some very smart hippos in there too.
MORAL OF STORY: don't let children play by rivers known to be homes of crocodiles.