Speaking to children is not hard. Children are little and inexperienced, however, they are not stupid.
Talk with your children much as you'd talk with any other friend. Use words that are appropriate for the child's age and take time to explain ideas.
Answer the child's questions and ask some of your own. Questions are a good way of communicating.
Don't be afraid to answer "I don't know." When you don't know be willing to find out. "I don't know, but the answer will be in the encyclopedia. We'll go look it up in a minute." is a wonderful answer to many tough questions.