Home theater setup recommendations?


My daughter, married, middle aged, has asked me to help them set up a decent home theater/listening room in their new home. I've been an audiophile since the 70's......before I even heard the term, so I'm a pretty competent 2 channel guy and I understand room acoustics............That said, I never really ventured into HT. I simply went with my 2 channel rig and was perfectly happy with things that way.....still am.

I guess my questions are..........How  important is multi-channel for movies vs audio.............dumb question, I'm sure, but it's not my area, so I'm asking...............and any recommendations for a decent, not UBER, sound system primarily for movies. They're willing to spend some money to get a nice setup, but not the kind of coin that many here likely would. I'm thinking used gear, speakers, subs, receiver or amps. Something that will sound good, but not require a second mortgage..................These folks think that hanging 4 inch Bose speakers in the corners of the room with a micro, wanna-be sub is good sound..............I want to "enlighten" them..........thanks
shadowcat2016
Is it smart to go from 0 to 100 in one step?

I’d keep it as simple as possible and lean strongly towards lifestyle factors. [Given your repeated mention of Bose satellites as being a reference for them]

Nothing at all wrong going with a well reviewed higher end soundbar (that is amplified and accepts ARC and has sub outs). If you want to be picky, then ignore the included sub and add a smallish (10 - 12 inch) audiophile approved sub.

Seriously, keep it simple. They will appreciate it and can move to the next step later, should this one take off. All the best.
Or get them Bose surround sound system as this seems to be a reference system for them and for lots of people (non-audiophile crowds). 
david_ten has a suggestion, but I would like to point out that a soundbar is somewhat of a "final solution". You really can’t upgrade or expand that solution. It is designed for people who want a easy solution to just putting sound out for their tv. Soundbars can sound good, but you are restricted on speaker placement and you won’t get the imaging that you can when you can place/position your speakers manually.
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Also, we didn’t talk about speaker cable. If you go to usaudiomart site and do a search for "audioquest slate", there is a guy that’s selling a pair of 8 foot + a single 4 foot cable (for center). He’s asking $159, which is great for the value of this. Don’t know if the lengths will work and it means that the receiver needs to be very close to the center speaker, but it’s a nice option.
Naim Muso will sound much better than soundbars but it is meant for listening to music only, not for watching movies or tv shows. And it isn't multi-channel setup but it is very simple. Thought I just threw it out there for you. 
@caphill  Agreed, and a good option as well.... but as you point out, it lacks multi-channel (minimally three or 'virtual' efforts beyond).

I have a Muso and it works well for stereo playback from the TV, but offers so much more for music listening.

An even simpler option would be to choose a TV with an integrated soundbar like LG's OLED series or, better yet, one of the OLED acoustic panels from Sony. The Sony's sound more than terrific (I have the LG with integrated bar) actually fantastic for TV / Movie sound. AND it's coming from the screen so placement has greater reality.