Item designed for small child (4)
I believe the answer is:
mite
'small child' is the definition.
(informal term)
'item designed' is the wordplay.
'designed' indicates an anagram (designed with the letters).
'item' is an anagram of 'MITE'.
'for' is the link.
(Other definitions for mite that I've seen before include "Microscopic animal" , "Arachnid; small child" , "Tiny contribution, child" , "The widow's small offering" , "Microscopic creature" .)