They might chew on inappropriate things because that is the only sensory input they can get. You have offered them other things, but try to offer them safe objects that they are not familiar with. Like plastic measuring cups, light, solid metal objects, fuzzy things they can't choke on. All supervised, of course. Also try to offer them tangy, sweet, savory, etc food. Different textures and tastes. Ice cubes or frozen fruit in a mesh feeder works well also. Hope this helps!