Just musing over this as took DS's little friend out to the cinema today. I was offered money by the parents but as I have asked the child to come along as a guest my view is that we are happy to pay.
It's not huge amounts we are talking about, childs entrance to the cinema or the cost of a soft play and we have only one DC and both work so have enough to cover it. Plus DS really enjoys it when he has a little friend along so it's a bonus for us.
I don't expect parents to reciprocate although it's nice if DS can go round and play at their house.
It's not a major issue but just wondered what other people do in these circumstances.