Fools the youngsters (4)
I believe the answer is:
kids
I believe this clue is a double definition.
'fools' is the first definition.
(kidding is a kind of fooling)
'the youngsters' is the second definition.
('kid' can be a synonym of 'youngster')
(Other definitions for kids that I've seen before include "They make immature butts" , "fools" , "Teases" , "Young goats" , "Youngsters" .)