If you ask my kids where they want to climb in the Bluies, as we did Saturday, you'll always get the same answer, Dam Cliffs! Too cold for swimming? Not if you're 9 or 11. Careful he may want you to join him cause the cliff jump requires company.
My first climbing experience was at Barrenjoey, stumbling along the cliff top, dangling ropes of climbs. It's good for that. There's bouldering on the beach (kids love it) and more swimming!
Mt Boyce is good for what you want except all the other suggestions have 10-15m cliff faces instead of 50m. You could always lower him on one rope while he abseils the other (kids love abseiling too) then bring him straight back up. You need to know how to set up a pully for hauling hard bits. Lots of potential for problems here...
I've never fumbled around the top of Mt York setting up top ropes. I guess it's a lot like Mt Boyce with less height and being able to walk to the bottom reduces the risks.