My kid sometimes gets gift cards of $25 or even more, and it's always weird to me that it's so much.
Especially because my kid doesn't get invited to birthday parties, so he doesn't get a chance to *give* gifts.
Of course, the attendees at my kid's parties get the "gift" of a day at the paint-ball place, or an afternoon of running around a huge room with Nerf guns. Which may be influencing the decision, I suppose. So I think I might have been more likely to go with $25.
In the matter of the spending money, I can see a scenario in which I'd arranged a trip like that with a few activities that I was funding, but knowing that the kids might want to do other stuff, or buy stuff, or whatever. So while they'd have a full party without bringing money, they might also want to bring some for their own reasons.
I don't think I'd be required to provide souvenir money and every activity the kid wanted to do.