It’s fun and very compulsive. Just doodling with colour! It also creates a lot of fine black dust so cover the table with some newspaper.
It’s something the youngest child will enjoy although you will probably find they have finished in five minutes whereas you have only just started!
The general rule is, the younger the child the larger the surface and the sturdier the tool so something like a spoon works well as they can grip the bowl and scratch with the edge of the handle. They also need a large piece of board or thick paper and supervision although it’s much more fun and better for them if you do one yourself.
I’d recommend trying out different techniques and tools on small pieces of paper before launching on a masterpiece!