A good surround system should allow the audio to be kept down lower and sound cleaner than if just the TV speakers are being used which means you don't have a squawking box killing your (and your kids) ears, but just good clean sound.
If you really want to do this, then the HDMI splitter for the cable box is the way to go and you will still want a decent universal remote that will turn on the TV without the surround system and put it on the proper input. But, I've got a couple of kids and they have listened through the surround system for years and it has always played back just fine after I taught them to keep the volume down to a reasonable listening level.