Just say something to them when you show up for work every day - "So, you'll be back before 6pm tonight? If you're going to be later that's okay, I'm just trying to plan my evening." They should then tell you when they'll be home. If they show up late that night, say something the next time you're there - "I just wanted to check in briefly with you both. I am proud that I'm able to provide a lot of flexibility for the families I work for, but it is important that I know what time I'll be relieved at the end of the day. Recently you've been coming home late quite frequently and I just wanted to ask that you make a real effort to figure out what time you'll be home so I can be prepared. If you need me to stay longer, please call and ask with plenty of notice as I might have other things lined up that I'll need to go to."
Just be direct.
This. Since they know you want extra hours and you've never said anything, they have no idea that this is bothering you.
DD and her friend do the same activity. I take them to the practice during the week and the other mom takes them to the practice on the weekend. In the beginning, the other mom would call ahead and ask if it was OK to bring her child back to my house afterwards so she could stay late. It was fine. We did this a few times and now she'll call and say I can pick X up this time. We've done it so much and I am/was being completely honest that it is no problem for X to come back to my house. The girls do homework and have fun. I don't expect the other mom to have to word her text as asking me each time. If its a problem I will say something.