I had to Google this answer because nothing came to mind when thinking of Disney + Quiet.
Here are five restaurants at Walt Disney World with a calm atmosphere.
Coral Reef Restaurant – EPCOT. ...
California Grill – Contemporary Resort. ...
Kona Café – Polynesian Resort. ...
Mama Melrose's Ristorante Italiano – Hollywood Studios. ...
Liberty Tree Tavern – Magic Kingdom.
My two cents...
Storybook Dinner can get a bit noisy when they do a special song. Everyone claps but it's not crazy or anything.
For a Character Breakfast I echo the Garden Grill breakfast idea.
Topolino's is not really that quiet but it is a beautiful environment. Characters sing & dance to a song every so often and it can be loud.
Grand Floridian Cafe always seemed calm to me but maybe it was the Victorian environment?
Narcoosees
Victoria + Alberts comes to mind the same as Monsieur Paul - but the prices are steep