Buy a 5.1 receiver and simply connect 2 speakers and a sub. The owners manual should show how to do this. You can add speakers when ready.
Every receiver is different, so it's best to look at the owner's manual that will show exactly how to connect just two speakers and the sub. Some receivers, like my Yamaha, have multiple Sub connectors (I use the single RCA Sub-Out) but some receivers seem to have two. In the case of two, buy a Y-Cable. Some subs, like my Infinity Powered Sub, has two RCA connectors for the receiver, but the Y-Cable solves this.
Cameras that make great holiday gifts
Let them start the new year with a step up in photo and video quality from a phone.