We did a bit of a tour in East Kent and Thanet when the duck was 1 and in the seat on the back of my bike which was nice, mostly on quiet roads and the sea wall. We started at Whitstable and rode to Thanet then Canterbury.
2 years ago when he was 2, we rode from Mildenhall into Suffolk, which was beautiful, although circumstances made our first day longer than I would plan. Suffolk has loads of lovely roads for cycling, as does Norfolk.
Another that I have thought of doing is around the Cuckoo Trail and Pevensey levels. I think that we might do that as a first mini tour with the lad on his own bike, because I think we can do really short days.
I have thought of doing a tour around the Tissington and High Peak trails (and the others in that area), but there is a bit of a problem getting there because of the lack of railways. Plenty of campsites though.
Do it, it's great fun!