A major challenge and goal with 2's and 3's is teaching them how to act in a more formal group teaching setting. Remember this and do not let yourself become frustrated. Your work with the children now will make way for easier teaching as they get older. Reviewing simple and clear classroom rules and praying for God's help with the children at the start of each session are great ideas. During the rest of the session, you may want to refer back to the rules in praising the children when they are keeping them well or when addressing problems.
Teach, Take & Tell incorporates basic rules into a chant that is memorable and fun.
Curriculum 
Class Rules Chant