After months of reading this board-here is the schedule I have come up with for our two families traveling together. 4 adults and 3 teens. Note-the adults have all been on a short trip to London once before.
Arrive London mid day Saturday-will be jet lagged. Just planning to walk around the parks, etc. Staying at Ruben's at the Palace.
Sunday - Tower of London and ?? Would also like to take an evening Thames tour.
Monday - National Gallery and St Paul's Catheral. - Possibly a third site.
Tuesday-renting cars at Heathrow-driving to Blenheim -then on to Burton on the Water. Staying at the Dial House.

Wednesday -1/2 day Burton and the Slaughters. PM driving to Thornbury Castle outside of Bristol. Want to get to Thornbury early afternoon to enjoy our stay overnight.
Thursday - 1 full day in Bath. Staying the night at the Bath Paradise House.

Friday - Stonehenge in the am. Returning our cars to Heathrow. Looks like we will fly to Paris - had org. wanted to take the train to Paris - but as we are already at Heathrow - should probably just fly.
Saturday, Sunday and Monday - in Paris.
Staying at the Duquesne Effel Hotel.
Tuesday - flying home from Paris to SFO.

Am I making any big mistakes? Please let me know. Thanks.

HUGE, obvious mistake right off the bat....afternoons should be for the teens to go shopping, hang out, go to the parks, in other words-- to be teens. Too much sightseeing, I really think you are jumping around way too much considering that the logistics include two families.

I think it is good idea to do something non-stressful like the hop-on hop-off bus tour the first day. You just sit there and get a taste of London. Evenyone gets to figure out where the attractions are located then your are set for your touristing later on. As many times as I have recommended this on this site I think I should start getting a kick-back from the bus companies.

I appreciate the feed back-but I don't think that you are seeing that we only have 2 full days in London. I have only listed 3 must do sites - how can that seem like too much? I figure we set out for 1 or 2 sites a day -then we will have time to see others as they interest us. Trying not to cram in too much - I want to have time to wander. Thanks.

How long do you want to spend at each of the sights on your itinerary?
I could spend most of the day at the National Gallery.
I would skip the Tower, or just drive by it on the on/off bus and go to Covent Garden with the kids and just walk there..I am not sure that teens would be interested in the Tower, a friend with a 12 year old went last fall and he was bored.
But, I bet they would love one of the Jack the Ripper tours !

Walk from Buckingham Palace (try to see the changing of the guard if possible) then on to the National Gallery. Across the street from the Nat'l is St Martins in the Fields, a lovely church, they have concerts at night, and a Cafe in the Crypt. Then you are in Covent Garden, the kids will love it and so will the adults. Go to Neals Yard, good music stores for the kids to bring home some British rock.
When you are at St Pauls , you can travel on over to the Tate Museums, I love the Tate, the kids might enjoy the Modern..and of course, there is the Wheel.

That is all I can think of right now Hope it helps.

Scarlett's friend must be unusual. I have known very few kids that didn't really enjoy the Tower. But never mind.

My concern is with your whole itinerary.

You have 7 people in tow - and want to do all these one night stops. Everything, and I mean everything, takes a lot longer w/ a group that size. Even just getting B'fast served and checking out of the B&Bs will take longer.

Why don't you pick ONE location in the southern Cotswolds and stay for 3 nights? it would be a lot easier w/o out all the packing and moving every morning. And a well-sited town like Malmesbury or maybe Cirencester would be a decent base for touring the Cotswolds and visiting Bath.