It's fine to ask a child's friend to come away with you and expect some kind of contribution towards accommodation/food or whatever IF you make that expectation clear when issuing the invitation. You MUST do that at the point of issuing the invitation and it's not reasonable to do it just before going on holiday.
Inviting a child is different to inviting an adult to come away with you. Children are not financially independent and so if you invite them to something you expect to pay for them - if you can't afford to, you just do t ask them.........and all this, 'I'm not made of money and can't pay for other children'....well, no-one is obliged to invite other children to join them on holiday - if you don't want to pay for them, then don't ask them.
We have had other children come away with us. We have paid for the accommodation, meals out, food in the accommodation, days out and entertainment. Children always had some spending money with them - one or two had obviously been instructed by their parents to offer to pay for things like ice creams, but I don't think we usually accepted unless it meant a lot to them. Our DC have also been invited away on holiday. The hosts have paid for accommodation and other stuff too - some of those holidays have been pretty cheap without big expense for the host (absolutely fine) and others have invoked lots if expensive stuff - as someone invited, you go along with what the hosts plans are and enjoy anything offered in terms of food, outings etc, whether simple and free or fancy and expensive.
Op, if you didn't plan to pay for the guest, I don't know why you invited them, and if you wanted money,mid don't know why you didn't specify upfront - that kind of arrangement would be fine, if you made it clear and gave an idea if the rough costs involved.
You may now won a cheap or a very expensive holiday - the guest child's family really do t know and not having agree to pay for certain things, it would be very tricky to ask for money at this late stage - pretty rude as its now too late to pull out. I think you need to accept you will need to pay for food - if that means you need to eat in more to afford it, so be it. I don't think you need to pay for 17 year old nights out on their own, but if you go on a family day out, you should.
Next time, think ahead more and either communicate clearly unfortunately if you want a contribution (fine) or don't ask a friend.