MULTIPLE times?!
I'm not going to even go there...
I think you won't have a very nice time with the Otterbox cases. Since your kids have broken their screens multiple times, I HIGHLY do not recommend them just because while they protect the devices very well, it's just pretty much hard plastic and breaks easily. My dad broke his twice in one year and now just uses my old Speck case. They're too expensive to give to a kid only to break again and again.
They get dirty too, and sometimes the device actually breaks while inside.
Which iPad model do they have? You want something that's a little bulkier, like a bumper.