In reply to GregHood:
My 3 and a half year old has an Edelrid Fraggle II, which she's been using since she was about 2 and a half, maybe a bit older. She really likes it, and is able to put it on herself. I think her favorite feature is the clip in point at the back which she loves being suspended on a sling and swung back and forth.
I've taken her top roping at our wall since she was 2 and half, although just a handful of times. As others have said, you need to find a wall with lots of holds close together. I've mostly taken her to Transition Extreme at Aberdeen, which has a couple of good walls for her, with big buttons that light up and make a noise when you press them to encourage her up the wall. The only issue with these is that she will then spend the next 10 minutes pressing the button ignoring any coxing her down!
Don't expect them to be doing laps at this age. She normally has a bit of a play around, climbs a route, maybe two, then wants to go off and do something else or go to the cafe, then comes back for another climb later.