Choosing the wrong place for a sub is a risk but using some testing tools and a $100 sub will find the best place. It's a small investment for a mufti-thousand dollar sub system. I just put subs in front under my center channel. I I needed more, I'd just add more.
An IB sub using a manifold would accomplish a few things. 1. You can use a metal grill over it and keep the kids hands away from the driver. 2. No boxes in the room so there would not be any clutter. Very high WAF (wife acceptance factor). 3. Easy to build. 4. Cheaper than a retail sub.
What's a manifold? This. The drivers oppose each other so the forces cancel each other out. Crank up a driver and try to hold it while playing. It's like a jack hammer. But when they work against each other, there are no external vibrations. Scroll down on that link to the picture of the speakers and amps. See the 2 small openings in the wall? That's where the sub sound comes out. Put a nice grill over them and you will never notice them until you turn up the volume. Also, read the guy's comments on sound quality.
Prices - Fi Car Audio IB subs - http://store.ficaraudio.com/ib3-series/ $224 to $274. I'd go with 15's at least and use a pair of them.
http://psicaraudio.com - The IB subs are built on their Platform 1 system and start at $225. You can get all kinds of custom features that you need. David is a great guy to work with. And he learned his speaker building skills from Scott who owns Fi Car Audio.
Acoustic Elegance - $350 but these are hand built. Subs of this quality are using in boutique speakers selling for $50,000 and up.
Stereo Integrity - Yes, it's a 24" sub but you would only need 2 and it would perform better than 6 or 8 15's. Nick also has other options available.
This is not the complete list of subs that are suitable to use. There are literally hundreds available. At one time, I was going to try some older Cerwin Vega V-Maxx (orange surround) 15's but ended up selling them.
Amps - Behringer EP series or iNuke DSP series. The EP amp is just an amp. The iNuke DSP series adds Digital Signal Processing. EQ will make a good sub sound great. It will make a great sub sound fantastic. My sub was a boomy mess when I first built it. Now it's awesome. I use an EP-2500 amp with a Behringer digital parametric EQ. The iNuke DSP amps combine the 2 into a single chassis and allow fine tuning via a PC interface.