Listen to the word structure and diction level employed by the child. Try to use a similar level of diction, while attempting to see the situation from the child's point of view.
Listen to the child's response to what you say, and encourage him to ask as many questions as possible. If the child doesn't understand any concept that you're attempting to convey, chances are they will ask questions and attempt to gain clarity for themselves, without your prompting.
Use examples from situations in their lives to create parallel instances of the point you're trying to get across. Trying to explain why you can't always be there to play because of your work schedule, may be mirrored by suggesting that the child can't always play with their friends at any time because they have homework or chores around the house to carry out. Use hypothetical situations and situations from the child's real life.
Keep the message that you are trying to convey simple, and don't add unnecessary or details or digressions. If you feel that the child has grasped the basic concept, add more details at another time if you think it necessary.