S.S.
I have two under 4, so no pro from a personal experience perspective.
However, I did work on an education degree for most of my bachelor's degree program before settling into another field of study. Teachers are trained in effective discipline and techniques to maintain control of the classrooms they teach. I would highly recommend asking your children's teachers for suggestions - particularly since they work with the age group of your children daily.
One of my children is special needs and I know that can often scare people from offering help because there is a particular understanding about the specifics of disorders that they may not understand. Therefore, this might not be the venue to best generate a response to this question.
There are a number of websites relating to specific disorders and a majority of them have live chat rooms and message boards. I would highly suggest looking into that for your specific children's needs. One I saw recently recommended on his website was specialchild.com. I briefly breezed through it and it looked to have a lot of information on it.
In addition to the above, I have a friend who recommends the book "1, 2, 3 Magic" for behaviors. Haven't read it myself or used it, so I can't personally attest to it. However, she was struggling and did see a significant change from her two boys. Thought it was worth mentioning.
Best wishes,
S.