Boy and Grandad on train in great linguistic moment

Great moment on the train:

Pre-school boy with his grandparents. Non-posh. Boy (about 4) looks up at the moving LED sign about travel info.
Boy: What are they saying, Grandad?
Grandad (reading) "Customers are reminded to take care of their bags and possessions to prevent crime, when travelling."
Boy: What does that mean?
Grandad: Watch your bags or they might get pinched.


I've been thinking about this:
1. The boy initiates a bit of literacy. He knows it 'says' something. He's curious. He asks.
2. Grandad straightaway tells him exactly.
3. Boy thinks about it and realises he doesn't understand. He knows what to say to get clarification. (In the jargon, he has 'metalanguage' (i.e. language about language)) to get what he wants.
4. Grandad translates the sentence into everyday speech. (In jargon, he changes register) so that the boy can get it.

Wonderful stuff.