Children are opening boxes of yellow sand (4)
I believe the answer is:
boys
'children' is the definition.
(I've seen this in another clue)
'opening boxes of yellow sand' is the wordplay.
'opening' says to take the initial letters.
The initial letters of 'boxes of yellow sand' is 'BOYS'.
'are' is the link.
(Other definitions for boys that I've seen before include "Male children" , "Young males" , "Beach --, group" , "Lads" , "They will be themselves, it's said" .)