I have an 8 year old and a 4 year old and am looking at trying to find some easyish going walks which would be suitable. We like the Rivelin valley walk and today did the Porter Valley walk up to the Shepherds Wheel. Where would be a good walk to try next?

I would try a bit more of the Porter Valley from where you finished your last walk for a start. There's also the Loxley Valley. From Malin Bridge to Bradfield is just under 5 miles, and mainly flat with a regular bus service to get back.
Further afield, there's the Chatsworth estate, Rother Valley Country Park, Chesterfield Canal, Canon Hall at Barnsley. Good luck!