Too many variables right now to get specific about recommendations, but If I was given a chance I'm sure I could spec a theater/multi channel system that would cost the same as and outperform the components your choosing for your two channel system on two channel recordings. That is all I will say for now.
Building a system that addresses and solves the problems with the kids using the system and the dissappointing prospects of a partial theater, would probably go a long way for everyone in your household. Especially, if it can meet or exceed your expectations for sound quality.
Little tip, if you have a first rate contractor and he put all those studs perfectly 16" on center, even with double drywall your walls are perfectly tuned to a particular frequency use some odd studs between the 16" OC studs to break up the High Q mode you have created. Horizontal braces randomly scattered throughout the wall will help a great deal also. This will increase the effectiveness of the sound isolation and minimize how much the room effects the sound.
I don't know where you live but if you're near Baltimore I may have my store open in time to be of some help to you.