You don't have to give them back--period. If someone gives or gifts something to you, it becomes yours, and the gift giver has no further rights in or to the items. Keep the items, give them to someone else with child, throw them out, etc.--they are yours, and you may do anything you like. If they try to sue you, unless they can convince a court that they never gave you the items but only loaned them to you temporarily--which is probably very unlikely, since those sorts of items, from parent or in-law, are almost invariably gifts--they cannot win.