A.A.
I'm sorry M., but I think asking for a certain type of gift in tacky, no matter how you word it on the invite. No "gifts" should be requested on the invite to begin with. I know you just don't want more junk your kids don't need, but a person has the right to choose their own "gift" for someone else. I think the only thing you could do that would be appropriate is when someone asks you what the kids need/want, say "They are really into books right, that would great!" And then leave it at that. The person can then decide to get a book or not. Under no circumstances do I think you should ask people to give cash or gift cards, that will come off rude in my opinion. After the party, go through the gifts and ther ones they don't need donate to charity or return for a store credit if you want.