Hugs. Lots of hugs.
When I was 12, my special pet cow had difficulties with her first calf. We had to call the vet, he pulled the calf with mechanical assistance/leverage. We were lucky the calf survived, next the cow prolapsed her uterus. Vet stuffed it back in, sewed things up, and counseled us that while the cow probably wouldn’t need subsequent calves pulled, she would very likely re-prolapse. So she should mother and raise her calf, then wean him, and then never have a calf again- in other words, she was going to be beef.
My mom knew I was attached to my cow. I knew the cow could no longer pay her way, and would suffer if she had another calf.
Now, I also had a pony. I rode the pony, and really enjoyed him.
Mom let me know that if I really, really wanted to, we could sell the pony and keep the cow for my pet.
It ended up that my cow was put in the freezer (yes, I ate her knowing exactly what it was) and I kept the pony to ride. I was out of town when she was humanely euthanized (shot with her face in the grain bucket) and butchered.
But my mom did a lot to make sure I knew the whole story, the financial choices that made sense, and that if I REALLY wanted to, I could do something different. But it was a ‘budget’ type choice- either/or.
If your daughter is privy to an overview of the financial realities (not necessarily the specifics), and the likely outcomes of ‘rehoming’ versus euthanasia, she may feel less helpless. She may also choose to give up marching band for a paying job that lets you keep one or more of the ‘unadoptable’ horses. Or she might choose to really put effort into finding real homes for the unadoptable horses.
In any case, my opinion is that at 14, including your daughter at some level in this process (since she loves the horses) will be helpful to both of you.