Because those more money may not be enough to pay for more people that would need to be hired? While existing stuff may be wiling to stay 30 minutes more occasionally, they might not like the idea of having even longer shifts all the time.
You can scale the price to find a point where it's economical to offer to stay open while maintaining good relations and if the demand is consistent and heavy then scale that price appropriately or, alternatively, simply refuse to service repeat offenders if it's becoming a serious headache.
It's a bit of a hard question because the daycare may not have a legal way to stop holding the child, unlike fedex they can't simply return it to sender, so these sorts of contingencies might not be expected on opening your center, but you'll need to scale into them.
They can always call child services, and have DCFS pick up the kid. I think that is what one of the local daycares by me does, after a certain cutoff time (i.e., they can't be expected to stay open all night).
