I offered to look after a friend's two kids after school so that she could go take her eldest child somewhere and go onto to non-workrelated meeting. Not quite a playdate, but a nice chance to do her a favour, and my kids enjoy the company. She said she would pick up at 6.30 at verylatest because she needed to go on somewhere else in good time. I had her two children fed, coated, shod, and ready for 6.30 in the hall, to make life easier for her. Then she doesn't arrive. Not for any particular reason. She doesn't ring to warn me she is running late, say at 6pm. At the worst possible time of day I have to stand in hall with her anxious children, whilst my children are running amok and need to wind down, and she waltzes in 15 mins later 
AIBU that you should not have to keep other people's kids past 6.15 or 6.30 unless it is a properly arranged playdate, not something agreed that morning....
It isn't just a one-off btw - she is someone who is always trying to fit too much in. I just feel cross that she made me feel stressed and muddled to fit in with her super-organisation.