It doesn't sound "childish" and don't worry about making anyone happy with this tank except you and your family.
You can do a lot with a very docile community of small fish.
There are a few species of top dwelling fish. Hatchet Fish are one of the most common and African Butterfly fish are a unique looking but fairly common one also.
There are countless middle range fish to choose from. Also many of them are schooling fish. Keep in mind that schooling fish generally do not hide and prefer wide open areas over anything else. They find their safety in numbers, not in caves. Some are "fin nippers" and a large school of these amongst other completely docile species can be stressful on the docile fish. So read up and find a nice balance.
There are a few Dwarf Cichlids like any of the Apistogrammas or (Bolivian or German) Rams. These guys stay small (generally 4" or less) and although they display the intelligence and behavior of a Cichlid, they stay quite docile and mix well with community fish such as those I suggested above. The Dwarf Cichlids will mainly inhabit the floor of the tank dwelling in/near caves.
There are also any number of types of Plecostomus that stay small, show very interesting patterns and would be an asset to a tank like this.
For a while I used a 6' 125 gal tank as a Dwarf Cichlid tank and a Blue Dempsey grow out tank. It was also stocked with a couple of types of Tetras and other small docile fish. To be honest, I've never enjoyed this tank more than during that time.
If you stock your tank something like I have suggested here, I honestly feel the sump filtration system will be a waste. Sumps are well regarded for their ability to offer biofiltration, but even a fairly dense stock of these small fish will not compare to the ammonia produced by larger Cichlids. I think you would be much better off to spend your money on a quality canister filter (or two). It will also be easier to set the tank up to have silent filtration using canisters as opposed to a sump.
I also wouldn't bother with an under gravel jet system with these small fish. Their tiny waste is so easily moved, the flow of the filter/s and possibly a powerhead should be plenty strong enough to prevent waste from collecting on the bottom.
Also, just to throw it out there, I suggest considering sand as opposed to gravel. Gravel allows waste to fall down into the gravel and rot. Using a gravel vac is then pretty important and can be a bit of a pain to do regularly. Sand keeps waste on top of the substrate where it can more easily be pushed into a filter intake. It's also MUCH easier to siphon waste off the surface of sand than to do a thorough gravel vac.
Feel free to read up on the subject to get a more fuller view of your options, but most people side with Pool Filter Sand. It is readily available, inexpensive (under $10 per 50 lb) and the particles are large grained (compared to other sands) which makes them settle faster and less likely to get sucked into your filters.
　
Lastly, don’t limit yourself to what the LFS guy and I are suggesting. Put some hours into reading as this tank will be in your home for many years to come. Creating “the right” tank for you and your family can make this a wonderful family project… as opposed to that eyesore you paid too much for… Make Google your friend ;-)
