Take their books head on.
Gather all your kids' books from around the house and throw them in a pile in the middle of the room. (Yes, KonMari involves a lot of piles!) Are they too old for some? Uninterested in others? Sort the titles into "donate" and "keep" piles. Oh, and FYI: if your mini me has had a book for months and has never cracked it open, you can bet it'll stay that way. More is more when it comes to the donate pile.