I see what you mean about adjusting to unexpected situations! It can be really helpful to have some flexibility built into the schedule for those kinds of moments.
For us, we've learned to leave some buffer time during supervision duties so we're not too rigid with our timings. That way, if younger siblings need extra help or something unexpected comes up, we can adapt and adjust without it throwing everything off.
Also, my older child has developed a great habit of anticipating problems and thinking ahead, which helps us prepare for those kinds of situations. We've also found it helpful to have an 'Emergency Supervisor' on hand - in other cases that might be me, but sometimes the older child takes on this role if I'm not available.
Having these processes in place really does help reduce stress when unexpected things happen, and it's been amazing to see our kids grow into thinking critically about how they can problem-solve too!