You could take the whole gang, plus your own picnic lunch (which they allow happily, although you can also buy stuff there (hot fodd & snacks, but like most venues it will be more expensive than your own nibbles), over to Craggy Island Climbing wall. There are two actually - one out near Guildford and one in Carshalton. I've climbed frequently both venues where they offer instructor led sessions (including bouncy boys' birthday parties). The Carshalton one has a large bouldering wall and a huge indoor caving system to explore. A great day out for giggles, challenges and good physical activity. Beats stuffing their little tummies with junk food and watching telly all day, and it might just ignite a new passion in your little boy!
http://www.craggy-island.com/
The instructors are all qualified, insured and great with kids, whether they are confident kids or nervous ones. I've watched them in action when I have clmbed with my husband there over the years.
Whatever you choose - have a great day!