Have often thought this in passing, mainly due to my dm teaching me as a child and her being super strict.
Now mine are all at school this has come up a lot:
X's mum is a TA in ds' class. X and ds don't get on but my ds says anytime he 'tells on x' the mum takes her child's side.
Me being told about the the dinner ladies and what they have to do for my son at lunch. By a dinner lady at a social event unrelated to school and in hearing of others.
Me being told chattily about my son and his lunch habits.
Dinner ladies unable (seemingly) to be fair when their children are involved in incidents.
One helper taking my dd to one side as she came out of school to tell her off for her meanness to her dd. Has done this to other children.
They are just examples i know rather than guessing. But I think volunteering/ working is fine, just not in the same class or playground as your child....