Little fish: young child has swapped ring for one (7)
I believe the answer is:
tiddler
'little fish' is the definition.
(tiddler can mean a small fish)
I don't understand the remainder of the clue.
(Other definitions for tiddler that I've seen before include "Minnow" , "Small fish (colloq,)" , "Little chap" , "A small fish children try to catch" , "Small fry" .)