Possible Clues:

• "Old MacDonald hHd a Farm" refrain
• "... farm, __"
• Letters sung after "had a farm"
• 'Old MacDonald's Farm' refrain
• 'Farm' letters
• Kiddie's refrain
• Refrain in a song full of animal sounds
• ". . . had a farm, ___"
• Letters sung after various animal names
• "... had a farm, ___" (song refrain)