Correct sentence: A monkey’s favourite treat is a banana.
Why this is correct: The word "banana" is one single, countable thing, so we need the little word a before it: "a banana."
Extra notes for a 6-year-old: "monkey’s" means "belongs to the monkey." Also, in American English people write favorite instead of favourite. Both spellings are fine depending on where you are.
Try this: Say the sentence out loud: "A monkey’s favourite treat is a banana."