This may be obvious, but I learned it too late. No matter how much your child begs, do not give them a sip of your latte. Especially if it is just before boarding a cross-country flight.
Their little bodies cannot process caffeine and a five year old rocketing around an airliner at full speed could potentially be a hazard to civil aviation. You do not want to be responsible for the ensuing forced landing.
Trust me on this one, really.