I would add a clause to your contract that states if a parent fails to pick up on their scheduled time and this causes you to miss a paid outing that the parent is responsible for reimbursing you for money lost.

Unless it is in your contract you really cannot enforce it. So if she refuses to actually pay then there isn't anything you can do.

Seeing that your contracted times for pickup are not flexible do you enforce a late pickup fee? I would if it affects your group that much.