I see field trips as still being in school, this includes getting to and from. So just as if another parent couldn't take little Timmy out of class and walk off with him. Parents can't make a pit stop with little Timmy in their car during a field trip to run a personal errand.
But this wasn't a personal errand. This was picking her kids up at school. And, if it's like my school, there is a limited time frame in which you can get the kids or you get in serious trouble with the school.
I have some sympathy because I can see how the mom may have had a time frame in mind
3:00: Leave pumpkin patch
3:30 :Arrive Preschool and drop off kid
3:40 :Go to Elementary and pick up older kids
Then there is a delay leaving the pumpkin patch and she may not have noticed or may have noticed and thought she still had time to go to the preschool and then the older kids school.
Then she gets in the car and realises that there is no way she will be able to get to elementary school in time if she goes to the preschool first. So she tries to call and can't so fetches her children.
I mean I know that I often get in the car (with young children) and realise that it 10 or 15 minutes later than it should be because the time evaporates.
I wouldn't see it as a big deal if my kid arrived later in a convey.