Childish thing to say to a black sheep (3,3)

'childish thing to say' is the definition.
I don't know anything about this answer so I cannot tell whether this works.

'a black sheep' is the wordplay.
I cannot really see how this works, but
'a' is present in the answer.
'black' could be 'b' (as in pencil classifications) and 'b' is found within the answer.
This explanation may well be incorrect...

'to' acts as a link.

Can you help me to learn more?