Trying to sync schedules with new job and custody arrangement

So I got promoted which is great but the hours are more unpredictable now. My kids are with me Mon-Thurs and I'm worried I won't be able to pick them up from school on time some days. My ex is already threatening to take me back to court if I can't maintain the schedule. I don't want to lose this opportunity but I also don't want to lose time with my kids. Has anyone dealt with this? Should I hire a nanny or ask for a schedule adjustment?