I understand that kids can be hard to predict and harder to control, but if he can't get in and be strapped in his seat, he can't fly, (the pilot can't even legally take off) and that is not the fault of the airline. Beyond that, they also have a duty to the rest of the passengers to not allow one to annoy everyone else. If the kid's not going to settle down, better to find that out while the plane is still on the ground than when it's at 30,000 feet.
Ms. Kulesza is appreciative of the response, but believes she could have calmed her daughter down, if given the chance.