Bill joined by little son and little girls (5)

'little girls' is the definition.
Although both the answer and definition are plural nouns, I can't see how one could define the other.

'bill joined by little son' is the wordplay.
'bill' becomes 'chit' (chit is a kind of bill).
'joined by' means one lot of letters go next to another.
'little son' becomes 's' (abbreviation for son).
'chit'+'s'='CHITS'

'and' is the link.

Can you help me to learn more?