Given the way you want to set it up, don't rule out a soundbar.
Turn the built in speakers down to 1 on the volume, it will be like they are off but won't show a "Mute" thing on the screen.
BTW, you might try running the digital or optical out from the TV, so it plays whatever the TV is playing - OTA, game, etc. - to hook up the receiver for when you want to use the real system. That way you get multichannel if it's available.
IMO you're wasting your time and money. Run the out from the TV, when you want real sound turn on the receiver and switch it to TV as the source. For the kids, use the TV speakers.
Sometimes what people do in this situation is get a programmable remote that does all those things, so kids, babysitters, etc. just press one button. If you can find one that doesn't suck, and works with your hardware, that's another option.