"Din-din” is a child’s way of saying “dinner.” “Din-din” is cited in print from at least 1861 and appeared first in English (and not American) publications.
“Morris! Time for din-din!” is a line from a Purina Cat Chow commercial featuring Morris, the cat.
Informal dinner: orig., a child’s word
(Oxford English Dictionary)
[Childish or jocular reduplication of DIN(NER n.]
1905 E. M. FORSTER Where Angels fear to Tread ii. 47 ‘Din-din’s nearly ready,’ said Lilia.
1920 WODEHOUSE Summer Lightning xiii. 265 ‘Come along, Carmody. Din~dins.’ Hugo had sunk into a chair. ‘I don’t want any dinner,’ he said, dully.
1957 E. TAYLOR Angel v. 220 Must be time for dindins, madam.
2 November 1861, Chambers’s Journal, pg. 284, col. 2:
“I want to see the lions have their din-din,” exclaimed Emilius John; nor was he by any means singular in that desire.
John Holdsworth: chief mate
By William Clark Russell
London: S. Low Marston, Low, & Searle
But though he repeated his question in different shapes, the child invariably answered “Noo, mamma got no din-din.”
“No dinner at all! Are you sure, my pet?”
March 1885, Our Waifs and Strays, pg. 6, col. 2:
“Sam,” said Lizzie, by and by, “what’s we going to have for din-din. Me wants mine din-din.”
Then the boy broke into a passion of tears.
“We’ve got nothing for dinner, Lizzie.”
By Mrs. Humphry Ward
New York, NY: Macmillan and Co.
“Mama ! din-din !”
August 1895, The English Illustrated Magazine, “A Brave Little Lass” by Mary Gaunt, pg. 468, col. 1:
“I wants my din-din! I wants my din-din!”
Where angels fear to tread
By Edward Morgan Forster
Edinburgh: William Blackwood and Sons
Well, din-din’s nearly ready,” said Lilia.
Morris The Cat: One Finicky Feline
by Marjorie Dorfman
03 Sep 2006
His fame grew quickly and Americans loved the finicky cat that was always in need of a “din-din” comprised solely of Purina Cat food.