My kids first went to Nowra at about that age. Along with the above suggestions I think you'll find most of New Nowra is suitable for them too. There's access at either end of Thompsons Point that drops you down to the easier ends of the crag i.e. Descent Gully Walls and Fossil Cave Environs both have a few easier routes.
I was half way up something on the descent gully walls one day, they were a similar age to yours, when they both started screaming in agony out in the bush somewhere. I thought their Mum was going to drop me. The little cave they were playing in was home to a wasp nest. They were both stung, multiple times ,simultaneously. If you let me know when your going I can have them show your kids where not to go.
I'm sure they'll appreciate it if you camp at the ski park, canoe accross the water, swim in the water and maybe even wander through the animal park with them. You could camp at the animal park but you won't get a canoe.