Speak to your children as you would speak to an adult. Not with adult language, but in a non-condescending, respectful way. Teach your children that you respect them and consider them as equals in inportance. This will build their self esteem and confidence.
Interact with kids on the same level. Crouch or bend down and speak to children on their physical level. That way, there is an even communication without the natural dominance created by your height. Your children can learn that they are as important as you.
Give your children more attention when they are good rather then when they are bad. It is easier to ignore the quiet, well behaved times and interact more to your kids when they are being naughty or not doing what they are asked to do. However, this sends to them the message that to get attention, they need to be naughty. Young children must have attention from the important people around them - including you - and they will do anything they can to get that attention. Even bad attention (being told off or yelled at) is better then no attention in their eyes. Give them more attention when they are good and they will be good more often in order to receive more attention.