An "invitation" implies that the person who is doing the inviting will be doing the paying.
So, I think it's important to make the invitation specific to what you're willing to cover.
When we've invited our kids' friends or cousins to vacation with us, we invited them to join us at our destination where we would provide accommodations, meals, and admissions -- but that we would not be covering airfare, shopping, or treats outside of meals (such as poolside ice cream or fancy coffee drinks).
My reason for excluding airfare was not just to save hundreds of dollars, but also to avoid being stuck with worthless, nonrefundable tickets if someone decided not to go. And that's exactly what happened with someone, when that kid decided not go after all to avoid being away from home for over a week. Now that kid's parents have to deal with trying to get value out of the nonrefundable ticket.
I also think it would be reasonable to exclude theme park admission passes if going to WDW because that's a fixed, known expense. However, because the guest (especially if the guest is a child) has very little control over what restaurants everybody goes to, it seems reasonable to include meals as part of the invitation.
I would hope the parents of the invitee would indirectly cover other expenses (at least partially), either by reciprocating (inviting your child in the future) or by giving their child funds to treat everyone to something, such as a nice dinner. Of course, it also depends on their ability to pay. And they have no obligation to do so.