Paying to go to a kid’s birthday party..?

[hannneshy]

Some friends of ours havs recently booked a party for their six year old at a zoo. The invite says all accompanying adults need to pay their own entry into the zoo (£15 per adult)
(details slightly changed for anon purposes)
Is this a pisstake, or am I being unreasonable?
if this was me, I’d have absorbed that cost into the cost of the party booking - I would’ve covered it. Surely..?

It’s not a “public place” it’s a hired room within a zoo.
I’ve done drop offs at parties within a hired space when the birthday child’s parents stayed and there is an entertainer or party host for the kids.