diddums
noun
1
used by adults for soothing and consoling babies and very young children; hence, a childish endearment UK, 1893.
2
used for offering heavily sarcastic mock-sympathy to an adult or older child and deride childish behaviour or for suggesting that such behaviour or attitude is childish. Adapted from the nonsense endearment used to soothe very young children. This nonsense is expandable: 'diddums doodums dumpling den' UK, 1893