>>Latin Quarter is not far from the Sorbonne area <<

Latin Quarter IS the Sorbonne area

Yes, Paris is heaving with tourists in Aug.

We all know and can repeat the old chestnut "everyone leaves Paris in Aug" or "Paris is less crowded in Aug" but that's very misleading. There are loads of tourists and some restaurants, etc are closed. Tourist sites can be at their most crowded. I think Paris in Aug is one of the most crowded times to visit. Offices and commuter trains are less crowded but that's not relevant to most tourists.

St Germain Hotel
 

Not sure if it is the same hotel or not, but we stayed at the St Germain des Preson Rue Bonaparte in Oct and found it to be very nice. Location was spot on; 2 blocks from the Seine, close to many local restaurants and priced reasonably. There was a small cafe at the end of the block (toward the River) run by a young couple that was fantastic. We would stop there each night for either dinner, a pre-dinner drink or dessert. By the time we left we felt that we were leaving family. Also, there is a chocolate shop across the street (think the name is Richart) that is listed as one of the 10 best chocolates in the world.

We did 5 cities in 3.5 weeks. 2 Adults 3 Teens
London.
Paris
Barcelona
Rome
Amsterdam.

and our least favorite hands down was Paris. We found it to be smelly dirty and crowded.
All around # 1 winner was Barcelona followed by Rome which tied with Amsterdam.
Barcelonas beaches and sites are NICE. We did the hop on off bus package and worth the two day package. So much to see and do.
Romes rich history earned the full 4 days. No need for the hop on buses everything is very close and easy to access.
We spent 4 days in Amsterdam and it has VERY nice meusams and parks. We took a daytrips to other places. The public transportation system is par NONE and so cheap!

I agree on sticking with two locations. Next, try to book "open jaw" flights which land in one location and depart from the other. I then suggest picking up guidebooks for those cities and their environs and start researching which sites appeal most to you. I would ask your kids to study these locations as well and pick what things to do sound interesting to them.

Certainly a more expensive area with easy walking to the interesting areas of Paris. I prefer to be a little more south and east but of the four in your set, I think you have chosen the nicest area. You will need to cross the river to get to some parts so it is worth working out a walking route that is easy for it, but south of the river it is easy to get around.