Maybe it because among my group of friends the other friend wouldn't be able to go if she had to pay the room by her self so we would of split it between the adults. You all need some where to stay and would be the same price of the daughter wasnt there so
If the other friends freely OFFER to sub the friend's child, so that the friends can come too, then that's a nice thing.
Otherwise, it may be disappointing, but part of the territory of having children is that you no longer have as much free time, flexibility and spare cash available. Surely this is stinking obvious to everybody?
You also acknowledge that children, being people, will incur costs and, as children don't tend to earn any money themselves, these costs will fall to you as a parent. It's not necessarily a fortune, but most households with children do get child benefit specifically to go towards the costs of children, and a lot also get tax credits.
This is why some people actively decide not to have children, because they want to keep their free time, flexibility and spend all of their money on themselves - and this is 100% a valid life choice to make.
We have a DS, who costs us money - actively and in potential lost earnings. We would be better off financially if we didn't have him. Would we swap him for that extra money and freedom? Absolutely not. This is also a 100% valid life choice to make, and the one that we chose for us.
With costs that will be incurred anyway, such as petrol, it's probably a kind thing to do to discount the child and split it between the adults; however, all personal costs incurred by the child that wouldn't be payable if she weren't there are naturally payable by her or, more realistically, as she probably doesn't earn any money herself, by her parent.
Also, this might be missing the point a little, but as the show is based on a children's book, the chances are that the friend's daughter might get even more enjoyment from it than the adults will.