This is a very interesting topic. I have taught 4 and 5 year olds in my time, and they were either piano or violin. Of course, at their age, 30 minutes is way too long, but the parents want their kid to be the next child prodigy and complain if I come out of the room 3 minutes early. I teach kids that young for 15 minutes and play with them for the remainder of the lesson. I don't know about age three, but four can only handle 15 minutes tops.
I don't know if many will grasp technique at that age, but constantly being around the language of music will most definitely be enough. I think giving them an instrument to toy around on is a good idea. Vic Wooten would say let them make any noise they want to out of it shy of destroying the instrument (hitting, plucking, whatever). It's like a kid experimenting with their voice and the sounds it makes before they can make any comprehensive words. If you've never heard Vic's beliefs on how music as a language compares to any spoken language, check it out. Most of his clinics I've heard will touch on this.
This reminds me of something that my ethnomusicology teacher talked about in class. His infant son ran up to one of his hand drums, beat around on them for a while, and then played four even eighth notes. As soon as he heard that, he said YES! to the kid. It's much like the same way we praise a kid for saying a real word after all his gibberish.
Nick