School pickup schedule changing and ex is being difficult about it

So my company is relocating our office closer to my house (finally some good news) but it means I need to adjust pickup times on my custody weeks. I'm asking to do 4:15 instead of 3:45 on Tuesdays and Thursdays. My ex is saying this "disrupts the routine" even though we have aftercare that keeps the kids til 5:30 anyway. She's threatening to take it to the mediator. Is she legally allowed to block schedule changes that are actually better for the kids? Madison is 7 and Lucas is 9, they've both said they don't mind. Feeling pretty frustrated.