Rage!!! I just wrote a long reply to you here and my account logged out for some reason before I posted when I clicked preview I lost the whole thing.
Anyway here I go again, we did pretty much the same trip as this last year with two littlies, broke the drive up on the way down and then just bit the bullet and did close to a 12 hour day travelling on the way back.
Tips for the way down:
Papea Parc Le Mans this is a fantastic theme park with lots of rides for kids of different sizes. We stayed
here the day before which is a lovely park with lots of cycles including bmx and mountain bike loops if you are adventurous.
Then a short hop to Noyer Sur Cher where we stayed in a nice
aire on Rue du Port beside the canal. We have friend that run Experience Canoe Kayak here so we went on a really nice canoe trip on the Cher river with them the following day. The water is warm and not to deep so plenty of opportunities for swimming and splashing about with little ones.
Noyer Sur Cher is then only another short hop (10mins) from
Zooparc de Beauval
This is rated as one of the top zoos in the world and I'd well believe it. They have designated free camping car parking and allow overnight parking. We only booked one day tickets but if we were to go again we get the discounted two day ticket, the place is so big that we really struggled to see everything in one day especially with small kids. The bird show here is epic.
We made a beeline to the alps from there which is about a six hour drive. if you are going to the Chamonix area I cannot recommend Les Arc 1800 resort highly enough. there are tons of child specific activities there, climbing, mini high ropes trampolines, pony rides etc etc etc along with great hiking and mountain biking and also a fantastic public swimming pool with indoor and outdoor heated pools and a couple of nice slides too.
You can park overnight either in the very queit carpark at the funicular railway at the bottom of the mountain in Bourg Saint Maurice( this is also beside a really nice park) or you can stay in a carparks up in the resort. I would definitely recommend this option as it is much cooler at night and you wake up to a cracking view of Mont Blanc,
the one we parked in was right beside a lovely playground for the kids which kept them entertained while we were cooking etc. plus had a drive over drain too.
On the way home if your leaving via cherbourg there is a really nice
aire beside a beach with a lovely restaurant in Hameau de la Mer that is only then 40mins from the ferry.
Hope this gives you some ideas.