Small child's slide (4)
I believe the answer is:
skid
'slide' is the definition.
(I know that slide can be written as skid)
'small child's' is the wordplay.
'small' becomes 's' (abbreviation - e.g. clothes size).
'child' becomes 'kid' (synonyms).
's'+'kid'='SKID'
(Other definitions for skid that I've seen before include "Uncontrolled car movement" , "Lose lateral control" , "unpredictable motoring event" , "Slide sideways" , "Swerve out of control" .)