Keep it simple and make heavy use of pictures and actions.
Nouns are good to start, you could start with "hello" but how are you going to explain that it is a greeting if they understand nothing?
You can teach simple commands by doing it with them, and combine that with pictures to help with simple grammar.
e.g "sit down" can become "sit down on the chair" or "sit down on the floor."
have plenty of activities and games, the studies say the average child's attention span is 10 minutes so look to change what you are doing every 10 minutes, my personal experience would tend to agree, change it up often.
Your general behavior is also important kids will pick up on your mood and alter their behavior, try to be bright and smiley with exaggerated actions, this is one of the parts that makes most people tired. They will follow and copy everything you do, so unless you want a bunch of 7 years running around shouting screaming "little S*(#s" to everyone they see control that language.