My family went to Kuantan, Malaysia recently. There were many hermit crabs at the shore.
Find out more on hermit crabs http://www.wildsingapore.com/wildfacts/crustacea/othercrust/anomura/hermit/hermit.htm
This boy was catching the hermit crabs for fun and he wanted to take them back. He shouldn't be doing that because this will deprive the hermit crabs of their natural habitat and the hermit crabs may die. The next time you are at the beach, I strongly encourage you not to do this.
Small crabs feed on particles in the sand, processing sand into small balls.