This sort of role-play has many powerful humiliation aspects, if the male is playing the child:
The older female is scandalized because she's fucking a child for pleasure- she's a pedophile, probably the most reviled sort of criminal in existence. She revels in the idea that she has sunk so low.
The older female is scandalized because she's being dominated by a young boy. She should be the matronly and authoritative one but she's so weak-willed and desperate (desperate is the key word) that she's made herself a whore for a kid not even old enough for a driver's license.
In short, getting fucked by (or even just subjected to the whims of) a child is a shameful thing. Nothing illustrates powerlessness and loss of authority like having to service the needs of a 12-year-old boy.
That's why this works.
- FS