The Funplex is a great place for kids to have a fun time. It's a soft play establishment where they basically run riot for an hour, leaping onto soft landings, swinging on ropes, and causing general mayhem in a safe environment. The Funplex also caters for Birthday parties and the like and charge about a tenner a head (for the kids).
Parents sit quietly and drink coffee and tea in the area for grown ups. You won't find this place by accident. It's right at the back of Bevendean. Definitely best to travel by car. There are buses, but a bit of a walk away.