“Why did the boy eat his cash?”/“Because it was his dinner money.”

“Dinner money” is money to buy dinner. The money itself is not dinner, but a popular riddle is:
 
Q: Why did the boy eat his cash?
A: Because it was his dinner money.
 
The joke has been cited in print since at least 1982.
 
         
Google Books
The Ha Ha Bonk Book
By Janet Ahlberg
London, UK: Puffin Books
1982
Pg. ?:
Why did the boy swallow 25p?
It was his dinner money.
 
7 July 1998, The Sun (London, UK), “Fun Spot,” pg. 20:
Q: Why did a schoolboy swallow a 50p piece?
A: It was his dinner money.
   
Google Books
A Man Walks Into a Bar 3
By Jonathan Swan
London, UK: Ebury Press
2010
Pg. 235:
Why did the schoolboy eat the cash? It was his dinner money!
     
Twitter
Laugh Your Head Off
@lyhotweet
Why did the boy swallow 25p? It was his dinner money! #haha #jokes #comedy
5:54 AM - 22 Dec 2010
   
Twitter
The Rise
@TheRiseofficial
Ok another bad joke, Why did the school boy eat a £2 coin?
Because it was his dinner money!!
4:13 PM - 2 May 2011
 
Google Books
New Jokes4Kids Joke Book
By Paul Gwilliam
Lulu Press (Lulu.com)
2013
Pg. ?:
Why did the boy swallow £1?
It was his dinner money.
 
Twitter
Living Sarcasm
@SarcasmLiving
Why did the boy
eat his cash?
Because it was his dinner money!
6:39 AM - 13 Apr 2018