Whether or not to cut London is a tough call; it is feasible to do it.
I’ve actually done London/Paris for a 10 day spring break trip years ago.
One reason not to do it would be the cost of transportation for 5 people, but if you bought Eurostar tickets (or easyjet or other plane tickets) well ahead of time, this might not be such a big deal.
Let’s look at the possibilities of cutting London. You could spend your time in the Loire Valley, then head west toward the Strasbourg area, perhaps stopping at Dijon along the way. I believe there is a TGV between Strasbourg and Paris.
By the way, after we visited the Loire Valley we actually headed west and north, and really enjoyed that part of the country: Nantes, Rennes, St Malo, etc.
edited to add: But we really liked the Strasbourg area, too!

London is a real favorite for me, so it’s hard to suggest cutting it, but your itinerary without it looks good, too.

As Cil says, getting to/from London and staying in London will be expensive for 5. Look for an open jaw flight (into London out of Paris or vice versa) so you don’t have two channel crossings to pay for. Getting around France can be expensive, too so compare to the cost of getting to where ever you’d substitute for London. Look for cheap leisure fares and other specials on Eurostar, the chunnel train, at www.eurostar.com

I think my son was about 16 when we were in France and UK. He much preferred London to Paris. He like Normandy, but preferred UK to France overall. I definitely preferred UK to France when I was a teenager.