My wife and I had five children in 8 years. therefore we were in many lines with babies and toddlers. One time with the seven of us in a long line at Arthur Threechers we noticed a harried mother and father with a rambunctious toddler who would not settle down. The father and mother tried to soothe him, even threatened to take him to the car. She asked my wife how we could keep 5 in line. My first thought was to cheerily say drugs to avoid my famous blunt tongue, but honesty prevailed. I said we tell the child he/she and I will go outside and come back when we are settled down. If we come back and he/she was to get rambunctious again, she/he and I miss dinner completely. No yelling or unfollowed through threats. The look from the couple was one of total annoyance and I let it go. If you do this, you do not have incidents in places like planes where it costs you dearly.
Why would the couple look annoyed at that? What you described works - telling a child the consequences, and following through. They have to know what to expect, and you have to live up to what you say you will do. As my grandmother used to say, "If you tell a child you're going to kill him, you have to do it." :)
I have 9 but I have never taken more than 3 at a time on an airline flight.
My son, who lives in Russia, travels back and forth frequently with his wife and their 4 children. My 3-year-old grandson is a perfect delight on airplane rides although I have been told that he is an absolute tyrant at home.
My kids could be very nasty brats at home but they were always well-behaved in public. Which is a blessing.