Hmm it is definitely a little bottom heavy. Right now your mids area is going to look pretty empty.
Cories aren't really a cleaning crew. They will eat food that falls to the bottom, but they don't eat algae or other detritus. They are SUPER DUPER cute though. My favorite fish.
Kuhli loaches are awesome too, not surprised your kid is interested!
However - if you're looking for the fish that he'll be able to look at most often, that will be the cories. The Kuhlis like to burrow down in the substrate, and they'll also hang out tangled up in plants (real or fake). Overall he may not see them as often as he sees the cories. But when he DOES see them, they are a treat! I would try to have him pick one but if he just can't, they will still fit!
You could get an angel if you took out the gourami, but then you can't do the chili rasboras as they are too small and will eventually be food.
Just as a note for you- Dwarf gouramis are really prone to disease. Frankly they are just not a very hardy fish strain. If you think your little man is going to be really upset when/if it dies you might want to pick something different.
For his fifth birthday my kiddo wants me to set something up just for him too, so I've had stocking plans on my mind as well. His favorite fish in our big tank is our Angelfish.
Also might I suggest a mystery snail or two? They are surprisingly interesting, much more so than you would expect a snail to be. Our kids love them and have named them all. They always run over to the tank to see what the snails specifically are doing. I wouldn't go with too many though, as they are prolific poopers.