Our school has a Little Ninjas program for ages 3-4. The classes are no more than 20-30 minutes long, and have a lot of different game-like activities dealing with balance (like walking on a wooden beam placed on the floor), focus ("Who can stand the straightest?"), listening to instructions (Run to that bag, throw two punches, and run back to the end of the line), appropriate behavior (Please sit with your legs crossed),etc. There's a lot of running around, and punching and kicking pads. No attempt whatsoever is made to actually teach "techniques," and there are no "partner" drills, but it does get them ready for entering the regular kids' classes when they reach 5 or 6.
In my opinion, requiring a dan level instructor to "teach" a 3 year old is a waste of resources. It makes about as much sense as requiring an MD to apply a band-aid. A well structured program with an instructor who sincerely enjoys working with young kids makes a lot more sense.