Just got back from two nights camping and 75km of cycling; in the end we took up your suggestion
@growingvegetables - we started from Brighouse on Friday night, cycled to Elland and camped there. The following day we cycled to Todmorden with a long lunch stop in Hebden Bridge - this worked out really nicely as Hebden Bridge had loads of good choices for lunch and Todmorden had the Cricket club for camping which was right next to a big park and the leisure centre for swimming. Sunday was ok - it was a long day of cycling and not as much time to stop and admire the view since we went all the way from Todmorden to Manchester Picadilly. It was however almost entirely off road and the roads we did do were a similar level of busy-ness to the roads he uses on the school run.
I did try and plan a North Wales route and spent quite some time looking at it, but it seemed that most of the campsites on/or near the coastal path don't accept tents... I may plan more roads for the next one as he's getting more confident/predictable/faster, but at some point I suspect the 3.5 year old is going to want to come too and we'll be pack to the off-road routes again