On the positive side of things, if the kid had been running around the lounge, being loud during preboarding, it could well have gone out like a light as soon as the engines started. Sometimes kids do get overexcited when they travel, but running around and wearing themselves out on the ground is sometimes a better option than keeping them coiled like a spring until boarding! Now whether the parents should have taken their little darling elsewhere to have his run around is another matter.