It seems silly that I have to tell them this, but on the other hand, I don't think I actually figured it out until sometime in graduate school, in my 30s. And that it applies in the outside world as well.
The calendar. It is your friend.
(Google calendar possibly even more so, especially for families trying to coordinate complex schedules. Shared Google calendars (that I can check from home, from work, and from my smartphone) have made our family schedules far easier to manage!)