My contract says dropoff by 8:30am as we have school dropoffs to do. Often we continue on for a walk or to the park from there. Wouldn't it be funny if you were gone from the house when the parent dropped off late without notifying you LOL!

Late pickups are $20 late fee here. One time 5 minute grace period, then $20. It ususally doesn't happen too often if the deterrant is good enough.
I would move the pickup time back to 4:30 and start interviewing for a new family.