I don't have many problems with parents picking up late and rarely ever have to charge a late fee. My families have a few different options:

a) no notice and pay $1 per minute after pick up time with a min charge of $10
b) notify me shortly before pick up time and pay $10 per hour
c) let me know asap and my oldest daughter will take over childcare and the charge is $5 per 30 min.

If the road conditions are bad then I do give my families a 15 min grace period just because many of my families have to travel at least 15 - 20 min to get here from work. In poor weather that time can increase to 1 hour.