The pickup line is a strange kind of stressful: nothing is actually wrong, but you are sitting still, slightly late, thinking about the rest of the afternoon, and your body still responds as though there is somewhere urgent to be. A short breathing pattern used while you wait can shift that before you are back in a car with a child in it.
This handout adapts the same structured breathing research used across the other breathing exercises on this site: slow, paced breathing with a longer exhale than inhale reliably lowers physiological arousal within a few minutes, which is the mechanism behind the Stanford trial referenced on the Box Breathing and Physiological Sigh pages.
Use it as you pull into the line, not once the car doors are open. Five rounds is usually enough to notice a difference, and it is easy enough to do without anyone in the car realising you are doing anything at all. If you have more than a few minutes to sit, there is also a short meditation written for exactly this kind of waiting.