I could only type 'no' in this topic to give you a simply brief answer, but that will be not that collied of me.
But I will not let you with empty hands, altough it is indeed not possible for kids to become mummies this way. If you have a mummy, let him or her released in your neightbourhood and when the mummy will have a relationship with another sim and they will have a baby (that is only possible if you don't control your sim) you will have a baby mummy if you make that family your active family after the baby has born. That's the only legal way to do it.