Any veteran early childhood educator will tell you that the hardest part of the day is not the lesson plan. It is moving fifteen four-year-olds from the playground back into the building without losing your mind.
Transitions are where behavior problems spike and where classrooms spiral completely out of control. The most common mistake teachers make is trying to talk over the noise. Yelling "line up" over a playground full of screaming children never works. You have to use nonverbal cues. A simple rhythmic clap, a specific doorbell chime, or turning off the lights instantly grabs their attention without adding to the chaos.
You also need to eliminate the waiting game. Do not have children line up at the door until you are actually ready to walk through it. If they stand there for three minutes while you find your clipboard, they are going to start pushing each other. Prepare the next activity completely before you end the current one, and keep them moving.