In reply to sbc23:
> Kendall is one of the most welcoming for kids that I've visited. They have several walls that are suitable and supervising adults don't pay (or at least not much)
> Steve
Funny, I found it the worst. The kids bouldering area lasted 20 minutes (flashed the 2 hardest circuits) for mine aged 9 and he still had three years to go before he was allowed in the upper bouldering zone. Apparently the clientele didn't want kids in the other sections. I was starting to question the wisdom of me £7.50.
To the OP, bouldering isn't ever going to be up to much for such little kids because they need similar climbs on the same line, so they can rainbow and it's pretty rare you'd find a nice selection of jugginess for them to get up.
A bit distant but the best I've seen is Nottingham climbing centre, a good little traversing boulder area you can use for a lot of training games and a roped training area where the grades don't change much between 4a-5a so it's really easy for little kids to rainbow. If you want to teach some skills rather than just bimbling it's excellent.
I hate to say it but Clip n Climb (from 4 years up) was incredibly popular with my kids, seasoned wall climbers with 5-6 years experience.