The first thing I'd do is boost the number of your skirts. They will act much more interestingly in a larger number - say at least 6 up to 9.
I also hate to mention this, but zebra danios aren't really appropriate, despite their small size because they are extremely active swimmers. And the WCMM are a temperate, rather than a "tropical" fish - so they don't really fit with a tropical set-up.
For young kids, livebearers are a great option, very active, very colorful, and they don't need tons of individuals to be "content". Platies come in several color morphs, which will keep the interest of the young ones, yet give them some interesting contrast to "similar" fish. Platys come in red, orange, blue and some with black. Mollys also come in multiple color morphs, yellow, silver, black, dalmation. Then there are swordtails, which are very similar to platies (and they interbreed with them). Then there is the guppy and/or endler's which come in another array of colors.
Each of these can be kept in groups of 3 (you seem to like that number) males (best to keep single genders unless you want a ton of fry that you can't get rid of).
If it were my tank (easy for me to say, because my kids aren't attached to any of these fish)... I'd do this:
6 black skirt tetras
3 male platies - whatever colors are most interesting to you - BTW, there are a few different color morphs of a "mickey mouse platy".
3 male guppies - Again, whatever color you find most interesting.
3 male endlers - slightly different look and a real challenge for your kids to spot the difference between the guppies and endlers.
6 corydoras - good active bottom dwellers, that will give interest to the bottom portion of the tank. (They prefer a sand substrate, and watching them sift sand through their gills in search of food is just fascinating to me. Here's an example of what I am talking about - 2012 Fish of the Year. This fish is about 1.5 inches long - panda corydoras.)
