Had this happen in Australia recently at two different theme parks.
One family did it twice (2 guys and 3 children) joined another person. First ride we were waiting as in line children's only ride so had to line up with little miss so she wouldn't loose her place and have others cut in front of her (as other parents were also lining up with their children ). The ride held 6 riders as they moved up and down on a mini giant drop. This lady with her 3 kids was in front of us we were behind her, her children were next to ride the ride as should have been my child.
Then the two guys turned up with their children talking on the outside of the barrier and then she says oh if they want to go together they can come with my children i only have 3 and then next think you know that had lifted the children over the barrier and my girl missed out being next to ride the ride as did the couple of children behind us.
We saw 1 of the guys again in a line we were lining up in and low and behold when it got close to the front the other guy turns up with said extra children and steps over the rope. From then on we made sure that those guys were not in the line for any rides we wanted to go on.
Next time it happened different park lining up in the heat when 3 adults and one child walk pass us and others to join a man nearer the front of the cue. The people beside me looked at me and I looked at them and they said "talk about rude" I said "you are telling me, I'm not letting this **** happen at
Disneyland when I go".
same line and a lady says excuse me as she pushes pass us, I say out loud to myself "you have got to be kidding me", she turns and says my husband is up there" ok fine up you go.
I don't mind the occasional cut in, but the deliberate cut ins irk the hell out of me.
At one stage I had to stand with my hands my hips to stop some people from trying to sneak pass us.