This is the playpen that we use..and as u can see,we have hardwood floors. Kai likes to climb on it…

This is the playpen that we use..and as u can see,we have hardwood floors. Kai likes to climb on it and when he does,it slides back. One side has wheels and the other doesnt. the wheels do not lock. what can i use to help it stay in place when he climbs on it?