If you choose to donate, you may want to open them first (and yes, I appreciate that may be difficult). It makes it much easier on the charity.
Personally, I would donate so that someone gets the benefit. However, doing so gives the in-laws the impression that you kept them - opening that door a crack. So they will continue to give the gifts and may even, as the children get older, contact them directly about the gifts. So, in this case, I'd send them back.