Sorry, but you do come over as the driving force behind this rather than your DH. I would have thought how much maintenance is paid is between him and his ex-wife. Let's imagine he based his original amount on half the original household costs of raising his children (includes housing, heating, water rates, council tax, insurance, bills etc as well as clothing food, trips, activities haircuts, presents, etc). Now his ex has had an inheritance, which is essentially a contribution from her (deceased) family towards her housing and helps her with half of those costs. If that enables her not to work many hours outside the home, that is up to her.
It seems unreasonable of you to try and take advantage of her inheritance by encouraging your DH to cut maintenance.
You may also be using horse-riding as a red herring here. Once maintenance has been agreed between the parents, as long as the basic needs of the DC are cared for, it doesn't matter if she spends it on horse-riding, x-box credits, swimming, petrol driving them to activities or reeds for playing the oboe. It is up to her and her children. Everyone has their own priorities. My DC also do a sport which others would consider expensive, but they get great enjoyment and personal development from it. I probably spend less on eating out, alcohol, pets, holidays and lease cars than other families who would say their DC couldn't afford to do it.
Apologies if I've missed it, but are you working full-time and contributing at least half costs to your own household? If not, is that something you could explore?