wider is better too, give room to put rocks in and still have room for sand in the front.
for you niece, i'd go with a cube type unit....aquapod or biocube.
here is why.
its a all in one unit....filter and lighting.
the lights are good for most softies....zoos, mushrooms, ect.
the hood makes for NO top offs. a tank with no hood will need daily top offs of fresh water to keep SG the same.
Just my opinion, there are down sides....if you want better lighting, or don't like the build in filter, no skimmer.
if i was setting up one for a niece, thats what i would go with.