I'd buy a good ereader for yourself and lock it up from the kids. I really don't think any ereader can take the abuse you're describing, they are still made of glass remember.
With one kid, it was easy enough to teach her to take care of things and she never broke any electronics; she was playing on my computers unsupervised under age 2. Actually I never had to teach her to be careful with it, it was just something she picked up from watching me on her own. But 5 kids playing and competing for one reader... it doesn't stand a chance, unless you're prepared to be there and supervise their use of it 100% of the time and have them sit in one place while reading.