You will be carrying admission card, Express Pass card, and maybe hotel keycard.  Adults should have ID, people might consider having some money.  Lanyards with pouches are a great idea.  For about $10 each you can get fabulous Harry Potter lanyards that look like the Hogwarts house ties.  They have cheap plastic pouches at the end of them.  Probably a bit of overkill for the kids, but I like my Witz brand clear, waterproof, hard plastic lanyard that I hang from my Ravenclaw lanyard.  Anyway, the point is, you can get a lanyard and pouch from a variety of places for a variety of prices, and they will be helpful.  Just know that the cheapie plastic pouches can and sometimes do break