To be honest, a larger tank is easier
to maintain. In a small tank, one mistake can cause a crash...you have more room for error in a larger aquarium. You can also be more lax about water changes since any wastes are being diluted a bit.
Of course, after a certain point, a larger tank can definitely be more work. But I'm talking more like 75+ gallons, not a little 20-gallon
. I find my 50-gallon to be less work than my 10-gallons and 5-gallons, in fact. It's also easier to clean (in small tanks it's so easy to knock stuff over and it's also annoying to fit the gravel vacuum, ugh!).
Maybe go for a 20-gallon or 29-gallon with a small school of hardy, undemanding fish. Look up white cloud minnows, cherry barbs, celestial danios, harlequin rasboras, and zebra danios. Many of the smaller tetras would also work, just stay away from sensitive species like neon tetras. Then maybe a few fish for the bottom--a group of cory cats or kuhli loaches? Or just a single bristlenose pleco
. All of the fish I've just mentioned happily eat commercial flakes/pellets, are inexpensive to purchase, not messy (the pleco is the messiest of the bunch, though), and tolerant of less-than-ideal conditions (not that they should be subjected to them but they can survive most "beginner mistakes"). Some of the species don't even need a heater (the danios and minnows, for example).
There are also brightly-colored "Glofish" that might be good for your daughter
. Glofish are zebra danios that have been genetically modified to have a gene for florescence. They're just as healthy as regular zebra danios but the bright colors might be fun for a kid's room. They look a little silly to me but hey!
They wouldn't need a heater or anything special, just a properly cycled aquarium with a standard filter.