Paper towel laid down on fine hole strainer and dry rocks put there. Water run over them and move to anothe container. Helper was playing with brush, and in fact examining each stone and brushing away the loose bits is part of this process. The paper towel is for catching rock dusk and small particles that can be used as filler in other projects.
Two going on three is a good age for learning all kinds of new things by doing, such as using a shovel to pick up gravel and move it from one place to another. Also for learning how to step with a bit more awareness by walking over gravel with bare feet.. Plus developing upper body strength. The learning is more important than the mess being made, which fortunately is easy to fix with the sweeping that would be needed anyway.