Pika,

The reserve water is excellent advice.

I use the square rule about starting on time.
2 people = 4 minutes late.
3 = 9 minutes late.
4 = 16 minutes late.
5 = 25 minutes late.
6 = 36 minutes late.

A spray bottle is great to cool kids down. Spray behind the knee, behind the neck, the face and the wrist.