"If you guess correctly, what I will do with him, I will return him. However, if you don't correctly guess his fate I'll eat him."
There are 2 possible answers:
1. The mother could say "You will open your mouth in front of the child". To eat the child, the crocodile must open his mouth, thus fulfilling the fate. Of course this doesn't mean he couldn't kill the child right then and not eat him.
2. The mother could say "The ultimate fate of my child is that he will eventually die". This is an absolute truth that the crocodile cannot dispute and must return the child, though a