No matter what the mother says it depends on what the crocodile decides to do. There is an infinite possibilty of options that can happen if she gets it right but there is only one if she gets it wrong. The crocodile said he'll eat the child if she gets it wrong. So she says the crocodile will eat he child then the crocodile will not be able to eat the child or she'll be correct. Given that the child will then be useless to the child he may give it back.