Especially children cannot carry heavy steel tanks and sometimes it is also harder for them to navigate them through the water. It is a good idea to start them off on a smaller cylinder. Usually, the 7l volume can provide enough air for a relaxed dive in the shalllows (depending on the depth and breathing rate as well). You can also combine two tanks to a twin set.
The Faber 7L Steel Tank features a handy cylinder boot. It comes with a DIN/K valve and is Nitrox ready.
Steel tanks have the advantage that they substitute some of the weight from your weight belt. This is great when you dive in colder waters, having to compensate for the extra buoyancy of your thick wetsuit.