Possible Clues:

• Nonsensical syllables, maybe
• It is said to be coming from young child
• Sounds like childish behaviour in mini going to lecture
• Childish language
• "Goo-goo" and "boo-boo" (2 words)
• Infantile vocalizations, and a hint to the starts of 17-, 25-, 37- and 50-Across
• Infantile conversation
• "Go bye-bye," e.g.