I think it depends upon the age of the people you invite. At 18 and 20, I would have their friends pay meals and park tickets. If they are a little strapped for cash, plan on some meals in the room. I never ask guests to pay for their rooms. If they can afford it (and some of my friends can't) they "repay" us by taking us out for a nice dinner or dinner show.
If it was a younger child, I'd probably pay for the park ticket, but expect the parents to send enough money for food, and of course, souvenirs.
If flying, everyone pays for their own flight.