You say nowhere to hot on the Med, except in heatwaves, Johndoh. Thing is that heatwaves are becoming the norm. There were 4 in France last year:
https://www.santepubliquefrance.fr/presse/2024/bilan-canicule-et-sante-un-ete-2023-marque-par-4-episodes-de-canicule-avec-un-impact-sanitaire-important
Above 35 it’s not much fun doing anything, even swimming in the local pool is unpleasant because it gets too hot as it doesn’t lose enough heat overnight, regular swimmers were giving up and getting out feeling unwell.
In Summer we avoid the Med and head for cooler areas. Anywhere above 1000m is a good bet and the Spanish North coast much more pleasant.
When junior was 8 he was really easy to please, loved camping (which I see you’ve already got covered, Mick) and simple things like exploring. It’s a bit young for Heide Park, Europa Park, Parc Asterix etc. because of the height limits.
I suggest a mobile home on a campsite on the West coast of France on a campsite with a kids’ club. Choose one with a British company with units on it and there will be other British kids around. I’m reluctant to recommend a specific site as junior is 26 now and things have no doubt changed.