Last week, I introduced my two-year old niece to "The Alligator King", an infamous early Sesame Street animated short from 1971. It's a wonderful little two-minute animation accompanied by a song about a king alligator, who's seven sons attempt to cheer him up by giving him unique gifts in order to wear his crown. She loved it and now makes her mom play it dozens of times a day!
Children's Television Workshop (now Sesame Workshop) worked with independent animators in the early days of Sesame Street to make these educational shorts which have an ever-lasting charming quality, at least to me. Bud Luckey, who went on to work on some of Pixar's most successful films, was the man behind "Alligator King", and he also did such other great animated sketches for SS like incredibly catchy "The Ladybug's Picnic" and "That's about the size of it".
Edit: Whoa, an original AK cel is out there for sale! Yours for only $50 shy of a grand.