I suggest that this wouldn't be a problem for you if you would just assume that this is happening when you get in line. Stop expecting that everyone thinks as you do. Accept that some parents will allow their children to do something else while they wait in line. You cannot change this situation. It is what it is. There isn't a rule that says either you or the other parent is right or wrong.
Would you rather be right or would you rather be happy?
This has happened with me and I've not even thought it was a problem. Usually my daughter and, now, my grandkids wait in line with me but I've seen what you've described. We don't usually wait in 30 minute lines because I don't want to stand that long. lol
After your SWH: I'm going to wax philosophical here. I'm 70 and do remember that rule. I don't remember a rule against saving a place for someone else. This is an example of how, as children, we think in black and white. Rules seemed simple. As we mature we become more aware of the complexity of life and, to be happy, we have to let go of the idea that a rule will cover everything.
One of the reasons raising children now is more difficult is because we are focusing more on teaching them to think for themselves while still trying to balance life with consideration for others. We're no longer focused so much on rules and etiquette.
I suggest that so many of us are unhappy because life just isn't simple any more. We have to see a bigger picture. Also, the world has become a bigger more complicated place than it was and rules just do not cover every contingency.