"Sleepy drink" is a time-proven beverage to have right before bedtime that will put kids to sleep very quickly, and they're effectively powerless to prevent it. Well, that's the story we tell them anyway. Add in some embelishments like, "we need to turn off most of the lights while you have your sleepy drink because it's darkness-activated," and "with sleepy drink in your belly you'll be asleep within ten minutes of putting your head on the pillow." Good potions work like that, and this one is delicious and doesn't have any added sugar before bedtime.
There's just enough time to brush their teeth before they'll be helplessly asleep. And it's so good they'll ask for it every night before bed.