Dh was picking me up from parents' evening the other week.
MIL gets wires crossed re: babysitting nights & is off out with mates.
Dh discovers this when he rings (kids already in pjs) to find out where she is. He is unable to ring me & say 'get a taxi' as the school where I work is in a mobile dead zone.
SO, he sticks the kids in the car & comes & gets me. On the way home (dc wide awake & enjoying the adventure), we decide we might as well nip into supermarket rather than leaving it until following morning. We zip round in 15 minutes, max - by now it's about 9pm. Kids straight to bed when we get in.
Honestly, too many variables to assume it's their customary evening routine!