A 'D' Birthday

This boy is the younger (by about 6 years) of two boys.  He's small for his age, too, and is babied (in my opinion) by his brother and dad, who are always in the lessons.

Boy's dad: You might notice a slightly different J this week... [babying voice] Because he's had a birthday.
Me: I thought so!  So you're 9 now, aren't you?
Boy [downcast]: Yeah.  [pause] But I want to be 8 again.  I liked being 8.
Me: I understand.  Not much you can do about it though.
Boy's dad: Maybe, we should not have any cake, or presents, tomorrow?
Boy: Could I have a 'D' party?
Me [in my head - a what?  Have I misheard?  Am I really getting so old and behind the times...?]
Boy's dad [after a slight pause, with a look of confusion on his face too]: A what? [phew]
Boy: A 'D' party.
Boy's dad: What's a 'D' party?
Boy: You know.  Where I get a 'D' and have to go back to being 8.

Ah.  So, failing at being 9.  Not a bad concept.