1. The mother has to guess. Restating what the crocodile's riddle is saying something the mother knows. She knows that if she's right in her guess the child is returned and if she is wrong the child is eaten.
2. The mother has to guess Correctly. Any guess will not suffice for the child's return.
3. If she guesses correctly that the child will be eaten, then the child must be eaten. Next (after the eating (assuming she was right)) the child will be returned.... Crocodile poop. Otherwise, she was incorrect and the child will be eaten with no return (crocodile poop).
4. If she guesses correctly that the child will be returned, then the child will be returned unharmed. If she is wrong the child will be eaten and will not be returned (as poop).
5. A smart*** could guess something the crocodile will do to the child that is neither eating or releasing. For instance the mother does not know but could guess (either correctly or incorrectly) that the crocodile will touch, change, smell, hear, taste, the child. If she is correct then that is what the crocodile will do: example, touch the child and let child go.
6. Removing the option #5 the best option is to hope that whatever mercy provoked the crocodile to pose a chance for the mother and child is the same mercy which would provoke it to relese the child. So guess that the child will not be eaten. Otherwise the best you get is crocodile poop.