In reply to JamesGo:
We took our 9 year old (and her friend) to Symond's Yat (routes like this one
http://www.ukclimbing.com/logbook/c.php?i=33118). One of us led the route (Trad) and set up top rope and abseiled down to clean.
That said it's about an hours drive from Bristol and you need to know what you are doing but not maybe the best bet for a beginner.
I think there is at least one VD at Cheddar gorge with a good tree on top that you can just walk to up, so maybe that would work (left hand side as you are driving up, past the pay car parks).
There are a couple of easy bolted routes at Woodcroft Quarry if you are happy to lead them (need quickdraws) and then set up as a top rope.
Although places like Undercover rock do kids sessions outdoors they have to be a bit older to take part.