For a good day out, though not circular I would suggest the Ceiriog valley. It would be a beautiful day ride for a family, and if you do a there and back there would be hardly any hills on a ride of about 35 miles. You could make it circular but you'd go over serious lumps!
Park in Chirk, 8 miles to Glyn Ceiriog where there is a shop, pub and park. You then get a gradual hill for a couple of miles before you drop into Trecieriog. 3 miles on is a Llanarmon D.C. with either the West Arms or The Hand Hotel.
You could then extend the ride to Pystyll Rhaeader waterfall (can't remember any hills) or ride up to the end of the valley.
Wonderful views, minimal hills and refreshment stops along the route.