@SRD - Theoretically one could attach the trailer to the rear axle of the tagalong, but apart from producing an unfeasibly long vehicle, such an arrangement would likely be dangerous. The torsional force on the seatpost of the main bike would surely produce an unstable ride, or even snap the tagalong hitch or deform/snap the seatpost itself. Braking on a downhill would be challenging, to say the least.
This raises the issue of what to do when my eldest grows out of the tagalong. Kiddyback tandem looks most likely, then towing a trailer is less of a problem (except for the braking downhill). Dawes do a reasonable one, or there's the Helios option if I'm feeling flush...
@Laidback - We did get a lot of comments as we rolled into camp sites. Most people expressed incredulity that we would carry so much gear on bikes (drivers/caravanners mostly); a few professed admiration (but with expressions that indicated they regarded us as eccentric/slightly insane); very few were critical except one lady who, while showing us to our pitch said "Backpacking on bikes with two young children? That's brave."
We met a handful of fellow cycle campers, who were all naturally very supportive, swapping tips on gear, tales of touring., etc. Also it was interesting how many cyclists emerged from motorhomes to come over and chat cycle touring. On the boat trip from Morston Quay we met a German family who were cycle touring with their three boys. They were all riding hire bikes though and credit card touring, with luggage couriered to their B&B/hotel each day. When they heard we were doing fully loaded cycle camping the father opined "Hmm... that's too tough!"
On reflection, I must be some kind of nutty masochist as I'm raring to do the same thing again. My wife is not so keen however. Next time I think we'll do less fully loaded touring and stay in one place longer, doing day trips without our bags. Haven't really considered France as yet, but looking at the Netherlands/Denmark/Northern Germany as possibles for next year.
My next step some weekend in the near future is to do a solo cycle camp on the wee Raleigh folder somewhere in Scotland...