But if neither parent can afford the trip, then the DC doesn't go, surely?
The whole point of CHild maintenance is to ensure that DCs are not financially penalised by the separation if their parents. It has created an expectation, from some, that the DCs lifestyle will never change, irrespective of any changes to the combined income of the DCs parents.
If the DCs parents were still together, and couldn't afford the trip, he wouldn't go, and the parents would not be socially vilified for refusing to go into debt, sell their car or skip mortgage payments. If parents are separated, then the NRP is expected to make up the shortfall - even if that means going into debt or sacrificing their basic needs.
We all know of cases where the DCs are wearing too small/scruffy/unsuitable clothing, yet one or other of the parents is prioritising leisure activities, but these are in the minority. But, in most cases, both the RP and the NRP are living frugally to ensure their DCs get the best they can afford.
My DD lives 50:50 split between me and her Dad. He pays me maintenance via the CSA (adjusted to take into account the no of nights DD spends with him). I am financially responsible for DD - I pay for everything she needs; uniform, trips, clubs etc. I decide if I can afford a trip for her, and if not, I decline it. I do not ask DDs dad to contribute any more. That is part of the deal of being the financially responsible parent - budgeting and saying no when necessary.