Talking to children or about children, there is a time for both.

Sutterbug Photography made a good point under reinforcement.
Children do pick up on things you don't expect them to. This is one of the many reasons why example is so important.
Most of the time, when you want to communicate something with a child, it is best to speak directly, simply, clearly, and gently directly to the child.
Sometimes though, children get upset and don't want to hear what you have to say. Sometimes they may start yelling or cover their ears. You might try speaking louder or even end up yelling and they just yell louder, blocking you out.
Try this instead some time. When you have a child who is acting in this way and you need to tell them something, such as setting a limit or letting them know what a consequence might be, turn to another adult or even another child and quietly talk to that other adult or child about the child who is yelling and/or covering their ears. Tell this other person what the consequence may be for this other child (by name) or what the child's options may be. Do this quietly but close enough that the child could hear if s/he stopped yelling or unstopped their ears. Sometimes when children don't want to hear what you are saying to them, they still want to know what you are saying about them, to someone else.