Wow, Sandy this may be the one time I would say that's too young for anything beyond giving them a light ball and getting them to hit it in the air instead of catching it. I would not run any drills at this age. Nothing more than games that teach them to hit the ball up. If at anytime they seem to lose interest, do not force them to continue as this will just build a resentment for the game. Instead move on to a different activity.

Get a very light ball for this, it doesn't have to be a "volleyball" with the correct pattern, any ball of approximate size and proper lighter weight will do.
Even a typical playground ball will do.

I would start by holding the ball out in front of them and get them to hit it up out of your hand. Do this holding the ball both low and high for practice serving and passing.
Have them stand in front of you with their hands together, tell them to be still, and toss the ball up so that it bounces off of their arms.

If they get good at these you can add a low net to stand in front of and get them into the idea of hitting the ball over it.

Hope this helps
Lots of luck.

Coach in most cases I would agree with you, except for the fact that I have a 2 1/2 year old that has proper technique for passing and hitting from being in a gym at pracice with me and my wife. I was thinking about the same thing in starting a "volley-Tots" program soon. 3 and 4 is a good age if there is general interest. Drills???not so much, But introduction to the game and working with a ball is always good.

They make a training volleyball for that age child. It is larger and like a beach ball but it is a volleyball. I would not use anything heavier because their bones are still growing and you can cause great damage. Please be careful.