My mind is completely boggled that so many people think it's OK to require an entrance fee for a party and to dictate what the entrance fee is.
Guess what? If you tell your guests to bring diapers, you're asking them to pay an entrance fee (a PP even called it an entrance pass). You should never ever ever ask your guests to pay anything. If they offer, great! (Yes, Virginia, many of us always bring a bottle of wine/chocolates/etc when we are guests.) But never require it. How rude!
(And yes, if you are hosting an event that says Bring XYZ, you are requiring it, even if you don't turn people away at the door.)
And never ever ever tell someone what gift they must bring. Whether it's a diaper, a book, or a platypus doesn't make any difference. Never tell a guest they must bring a gift, and never tell them they must bring a specific gift. WTF is this so hard to understand?
Just because it's a cute etsy invite doesn't mean it isn't rude and tacky.