ZBrushCoreMini is a free introductory program for the ZBrush family of products. It has the minimum features necessary for digital sculpting, in a UI that is designed specifically for the task. As such, it is a wonderful tool for kids and beginners alike.
Anything that you create in ZBrushCoreMini can be opened in ZBrushCore or ZBrush if you later choose to purchase either of those.
See the ZBrushCoreMini pages for full information.