My kids once did a 3x/week, after school activity that went till 7pm. Another girl (8yo ish) did the same activity and the mom asked if I could supervise her until a parent could pick up so they wouldn't have to pay the nanny. Fine - I was there anyway. Only the parent kept being late & I'd be waiting in the parking lot with 4 hungry kids who needed dinner & homework. When I could get hold of the parent, they'd say they were still 30 min away and they'd pick child up from my house. Which meant I fed her and entertained her for another hour, while doing homework and bedtime routine.
That went on sporadically for a couple of years till we switched venues and schedules. Now a 6 day per week schedule, all year, the child would try and find out what sessions my kids were planning to register for each season so I could be used as a drop off point, supervision, and drive home (30min extra out of my way) or pick up point (at their convenience). My kids would tell her the wrong schedule, the mom would register, and then be gobsmacked when I told her the coordinated plan she'd worked out wouldn't work.
She always approached it as "sharing" the workload but it never worked out that she did anything. She never asked ahead of time, after the first "supervision". She never, ever said thank you. Some kids I wouldn't mind, but the girl was a shit disturber who thrived on creating chaos with mine and was quite snide. My kids didn't want to see more of her than they saw at the activity. Sometimes I'd go to drop her at home and no one was there - they were grocery shopping. Sometimes, as the years went on, the girl would approach me herself and tell me she had no way to get home - they knew I'd be there. Occasionally I'd say no, we're not going home, and the mom would bitch about me to anyone who would listen.
I felt sorry for the girl but put my foot down as much as possible. Her mother was relentless. She would manipulate situations to put me in an untenable place - like leaving a child alone at night. She just could not understand that her family's schedule was not my responsibility or obligation. 3 of my own, a busy schedule, and chronic illness were exhausting enough.The girl is now mid 20's, very independent, seems a little more pleasant...and she still hit me up for a ride when we were at an event last summer. I can sort of laugh now.
It's made me able to fend off the normal CF's without breaking a sweat. I've "shared the work" with many people over the years of school and activities, sometimes doing way more of a share, but only when it feels right and it's appreciated by both sides.