The only reason I wasn't suggesting a single larger fish like a betta or gourami is that many of my customers who bought biorbs were NOT happy with the idea of a single-fish aquarium (I can't print most of their comments on a family website).
I don't think the flow in a biorb would bother any of the common bettas or gourami's.
However - right now I would avoid "dwarf" gourami's, as there is a virus going around most of the dwarf gourami wholesale population from SE Asia and they are not in good shape.
The only downside I see to tetras is that if you cycle traditionally (ie slowly without any "boosters") you really can't add a full school at once to such a small tank - just 2 (maybe 3) fish at first, then 2 or 3 more after a few months, etc. Which can lead to skittish fish.
However - adding a booster like Stability (from Seachem) or Safestart (Tetra) can make it easier to add more fish sooner. But I'd probably plan on eventually having a single school of one species, rather than a noah's ark of 2 of this and 2 of that, if your going with tetras.
Skipton's had a pretty good mix the last time I was there, but I was out sick this weekend so I'm not sure for certain what's in stock.
Off the top of my head:
Black Neon Tetras
Rummynose Tetras (kinda delicate to cycle with)
Red-tail Blue Columbian tetras (too big)
Ember Tetras (my first choice - bright red and peaceful)
Von Rio Tetras
Pristilla and gold Pristilla tetras
Congo Tetras (too big)
probably lots more I'm forgetting...
other small fishes in stock:
Celestial Pearl Danios
a few really nice "micro" rasboras including Boraras merah
Emerald Eye rasbora (Rasbora dorsiocellata
Norman's Lampeye (Aplocheilichthys normani
And probably a few more fish I'm forgetting.
I think the micro rasboras are small enough that you could keep 2 small schools, but they may be too small for a young child to enjoy.
Another option - add a little crushed coral gravel, add a little pH buffer to the water, place a few snail shells on the bottom, and put in a pair or trio of small shell dwelling cichlids.
As long as you kept up with the water changes, multi's would probably do fine in there.
Its new, its improved (its still a work-in-progress)