Umm, wow. My jaw dropped reading this.
There is no way I could ever accept people who would be fine leaving their kid 2 hours and 15 minutes late. They would be gone. If I couldn't reach them within 15 minutes, then emergency contacts would be called. If I couldn't reach the contacts, then children's services would be called. What if it was an emergency? They need to pick up your calls! Wow. Wow. Wow.
If you want to keep them, I would be charging a late fee of AT LEAST a dollar a minute, PLUS an inconvenience fee of whatever your fun evening would have cost (the cost of a movie/treats, your dinner and what you were paying your babysitter). If they balk at all, BUH-BYE. My time is precious to me. It is worth more than any late fee. If they are okay racking up the late fees, then it is not high enough. If they are late even 30 seconds after this, BUH BYE. Wow, just wow. That poor kid.