As BirdsofPreyDave said, one way to tell is if the individual has a military ID card. Active duty, reserves, retirees, and qualified family members all have one. If you're divorced, the child gets one if not living with the military parent. I've seen infants six months old with ID cards. If the parents aren't divorced, a child gets an ID card at (I think) 10 years old (at least, that's how old my kids were when they started to get them), and then a new one every four years or so.
Ex-spouses who were married for a certain number of years of the military person's career are also entitled to retain their ID cards until they re-marry. The number of years has changed over the years and my DH retired 11 years ago, so I have no idea what it is right now, but you could research that and find out.
That being said, if the child of a divorced military person is a minor, he/she cannot reserve a room at SOG because minors cannot enter into a contract, nor would they have a credit card, but you could call SOG and see what, if anything, they do to handle qualified children who plan on traveling with a non-qualifying adult relative.
-Dorothy (LadyZolt)