Your kids needs this time to grow up to be childs age or highter and than you can use them as you please
i personaly dont like that part of the game, but you can ask them to hunt for food and they might find you new nice slaves
Best thing you can do with slaves is Trait breading, some ppl there having fun in making "perfect slaves"
my way for "perfection" amuse me a bit better, but thats choice :)