Chewing Is the coating on grids safe for chewing?

I recently discovered both of my boys have been chewing on their cages. The plastic coating is coming off and I want to make sure if they're swallowing it...is it safe? and if not, (even if so...) how can I get them to chew on other things? They both have houses made of wood and straw, wooden and straw tunnels, and other chew toys. They also ALWAYS have access to timothy hay.