In reply to nick simons:
Have you considered going to Bowles instead as there is a better range of routes for the age range (6 to 12 years old)?
You may however be competing for the obvious gully route at Bowles with groups of kids attending the outdoor center there. The gully is about 20 to 30 meters along the crag from the carpark end, so should be off to your right as you walk up the hill.
Also at Bowles there is an easy slab at the far end of the crag, which would be a decent challenge for a 12 year old. You may also find some of the easier grade routes ok for the 12 year old too.
The crag, as you may already know, is a short walk in and has the added benefit for kids of decent toilets in front of the crag.
I used to take my kids onto the southern sandstone and would favour Bowles if they wanted to do roped climbs. They would have a go at the obvious slabs at Harrisons usually unroped because they really were too easy.
If I wanted to avoid any congestion at Bowles an earlier start was always preferred.
Good luck.