It's true, number of travel days is a crucial bit of info needed!
We have interrailed via Brussels a couple of times. The first time we stayed for a couple of nights but we weren't terribly keen on it. The second time we changed at Brussels and went straight through to Cologne for our first night (an easy change, plenty of trains), and then on to Berlin. Berlin is absolutely great, would recommend for all ages. If you're transport buffs, you could stop at Wuppertal for a ride on the Schwebenbahn.
No Interrail reservations are mandatory on German trains, but I would book seat reservations in August- we didn't and spent some time sitting in the corridor!
I wouldn't plan more than 4 or 5 stops at those ages in 2 weeks.
You could include some mountains- maybe stop at Innsbruck? (on the main line between Munich and Italy). Lots of good places in Bavaria- we really liked Nuremberg.
Then maybe cross Switzerland in a day? Again, no additional reservations needed and there are some amazing mountain railways.
Coming back via Paris would seem to make sense, and it's always a hit. In August Paris-Plage will be going on- lots of free swimming spots on the Seine and canals. We went to the one at Bassin de la Villette.
Strasbourg is an excellent stop between Germany and Paris- we loved it.