Jersey I imagine they'd be filled with the same gel that is used in our own freezer/hot packs. Surely they are made with the possible chew factor involved, but it's not something I would leave unsupervised. There may be enough of a covering that a dog couldn't access the filling without a bit of work first.
A good friend of mine has a cooling collar for one of her english bulldogs. She takes it to the odd show she enters him in, but also uses it at home on days like we just suffered through. She swears by it and it's impossible for him to get at.
I bet they'd enjoy having the pad to lay on .......... I would have loved one myself this weekend.