Collection I'm backing for little children (5)
I believe the answer is:
mites
'little children' is the definition.
(I've seen this before)
'collection i'm backing' is the wordplay.
'collection' becomes 'set' (set is a kind of collection).
'backing' is a reversal indicator.
'set'+'im'='setim'
'setim' back-to-front is 'MITES'.
'for' is the link.
(Other definitions for mites that I've seen before include "little children" , "Minute arachnids that infest animals and stored foods" , "Widows' contributions?" , "Small arachnids" , "tots?" .)