In reply to nickfletcher:
The Peak in Stirling has no minimun age if you are bringing them in yourself. If looking for instruction though, the child must be 5 or over. I assumed most walls would allow you to take them in at any age if they were under your care, just not for instructed sessions.
At this age, I think kids often get on better outdoors. You can just find some fun broken ground to scramble over and you just stick behind them to catch them if needs be. Indoor walls are usually too reachy and 'formal' for kids of that age.
If you do want to go indoors, TCA has a great kids area. Not quite on the scale of Sheffield's miniworks, but pretty good.