WWYD about siblings of DS' friends coming to his party?

[MrsSawyer]

Im having a party (in the kids play area of a pub) for DS who will be 4 in August, and as its in the school holidays (and its on a weekday) people have said, "we'll come but ill have to bring X" (younger or older sibling).

I dont mind them bringing their other children as I appreciate that it can be difficult but I also dont feel like I should have to pay for them to eat, party bags etc.

What can I say that shows that other siblings coming is fine, but they have to stand the cost of meals etc and there wont be a party bag for them?

OR...am I committing some sort of social faux pas by not offering to pay for them?

Please help

Not at all,I would expect to pay for a sibling, would also check it was ok to bring them too and would try my best not to. It's selfish to just assume you can bring siblings, if everyone did it would be overcrowded. Certainly wouldn't expect them to be fed and get a party bag!
I would say (if you are happy for them to bring a sib) "yes that's fine, but the venue charges per head for food so you will need to pay for the sibling". Alternately you could suggest they opt to just drop off the invited child (if you are happy to supervise).

Im more than happy to supervise, might suggest that and with it beng 2 weeks into the summer holidays, parents might welcome 2 hrs off