Girls of nine, perhaps, in a bus (7)

'girls' is the definition.
The definition and answer can be both people as well as being plural nouns.
Maybe you can see an association between them that I don't see?

'nine perhaps in a bus' is the wordplay.
'perhaps' indicates anagramming the letters.
'in' means one lot of letters goes inside another.
'nine' is an anagram of 'nnie'.
'nnie' inserted into 'bus' is 'BUNNIES'.

'of' acts as a link.

Can you help me to learn more?