Things are evening out better now.
My new mantra is "they aren't my kids, they aren't my kids."
Their parents absurdly want them home by 2, when they don't go to sleep then, and nap shifts to after 3 in a few weeks? Fine.
They're on a ridiculous sleep cycle that I don't think is healthy for them? Fine.
Their birthday party is a day when their parents know I'm out of town? Fine.
I have to care for them within the constraints. It doesn't matter if I think the constraints are a piece of shit. They aren't my kids. I can do things differently; nothing is going on that's endangering their lives.
Somehow, I've managed to scale myself back emotionally. I'm still finding a balance between shutting down entirely, and where I was. I easily cut out emotionally, especially when I know I'm going to lose something/someone in the future. But I think, in this instance, I might be handling it more neutrally.
I love those kids. But I'm stealing a little bit of my heart back, because they'd run off with too much of it.