Mom keeps showing up late to pickups and I'm losing it

Every Monday and Wednesday I pick up the kids at 6pm. Mom has been showing up between 6:15 and 6:45 the last 4 times. I have to stay late at work and it throws off my whole schedule. My kids are getting tired waiting around. I texted her twice but she just says sorry and does it again. How do I handle this without turning it into a big fight? Our custody agreement says 6pm sharp.