That totally sucks that your daughter is having such difficulty with her periods. My little sister had the same problem at a similar age and my mom got her on birth control. The only problem was that, as a twelve-year-old, my sister was horrible at remembering to take it and my mom wasn't always able to remind her either.
I have NO idea if this is even possible for a twelve-year-old, so I could be way way off, but I have an IUD at the moment (the Mirena) and it releases a very, very low dose of hormone slowly over time (I believe it's progestin?) and eliminates my periods altogether. It's pretty sweet, in my opinion. The only thing is I don't know if it would be recommended, or even possible, to insert it into a twelve-year-old, or even if your daughter would be okay with that. If not, maybe something to think about for when she's older. In any case, I hope she's able to find some relief soon!