Actually, I was making the assumption based on, "Will Southwest let us change the name to another child??" -- that kind of led me to believe that the OP was speaking of a child, for some reason.
I did think of the gender issue, but decided it was pretty unlikely in the "bring a friend" context, as most of the time that means that the "children" in question will be sharing a bed. I'd say that at least 98% of the time, a friend taken on a trip by a family will be of the same gender as the companion child.
IME, the only thing that an under-18 needs to say domestically is, "I don't have a license yet", and they will be waved through.