Barbara's out, that is, with the kiddies (6)
I believe the answer is:
babies
'the kiddies' is the definition.
The definition and answer can be both people as well as being plural nouns.
Perhaps they are linked in a way I don't understand?
'barbara's out that is' is the wordplay.
'barbara' becomes 'Babs' (short for Barbara).
'out' indicates putting letters inside.
'that is' becomes 'ie' (short for Latin 'id est', 'that is').
'babs' placed around 'ie' is 'BABIES'.
'with' is the link.
Can you help me to learn more?
(Other definitions for babies that I've seen before include "Infants" , "Very young children" , "Storks' freight?" , "Recently born children" , "Small children" .)