Backstory:Ex-DP and I broke up 7 years ago, he left for another woman, we already had two DC - DD1 now 10 and DS now 8. After being splitting up I found out I was pregnant with DD2, but ex decided to stay with other woman and they were married 2 weeks before I gave birth. 6 weeks after I had DD2, OW discovers she is pregnant and gives birth to twin boys, so EX now has three DC who are 6 years old, as well as OW already having a DD who is only a month younger than my DD1.
Well that was an info spill; now onto the real issue:
My DC stepmother not only has a high paying job but when her first DH died he left her a very considerable amount of money and because of this, as far as I'm aware, her and EX keep separate finances, both contributing a percentage of their wage into a house hold account and then whatever is leftover is their own to spend on what they see fit (I know this seems like a ridiculous amount of information to know about EX finances but how I know will become apparent in a minute) Ex is a firefighter and earns a pretty average wage.
SM spoils her DC (Her and Ex have had a further three DD's), they have the best of everything - toys,clothes, electronics activities. Her eldest two DD's are both in competitive dance and own multiple ponies that they compete with , something my DD's would love to do but will never get the chance as I can't afford it.
My DC spend every fortnight, Friday evening through to Monday morning, at their fathers and most weekends follow are the same; SM and her daughters go off to a dance competition or horse show, while EX takes DTS to whatever activities they are doing that weekend, all fine and dandy, except neither me or ex can afford for them to do these activities, so my children just have to go and watch their brothers 
This is how they spend almost every weekend with their father!
Naturally this has caused more then a few fights between me and ex, as I think it's cruel to get my DC up every Saturday morning to go watch their siblings do all these fun activities knowing that they would never be able to do them. Ex has said he would pay half if I payed half, as this is what He and OW do,because it's not fair on his wife and other DC if he pays the full lot for our DC. He doesn't seem to understand that I earn a low wage and cannot afford to even pay half.
This is especially hard as the children are all such similar ages.
EX does take them all to soft play, wave pool and movies ect after, which is fully paid for out of his own pocket after ( I know, what a saint), but that doesn't make up for the fact his children have been forced to sit around all morning watching their brothers have fun.
It just seems my children will forever be getting the blunt end of the stick because their father and stepmother have decided to have separate finances.
So, would I be unreasonable to ask their step mother to either fund or give permission to EX to fund these activities? After all it is because of her financial situation that my children have to miss out, as she has made it clear to ex that she expects him to split everything evenly between all his children, and spend no more or no less on our children then he does on theirs, and she could afford it or would that just make me bitter, jealous cow? How should I go on from here?