The agent being employed by US makes US at fault by stating the info, wrong or right. If the agent told the parent it was ok and booked the minor with connection, who do you blame? I agree about booking the flight with double connections and a change of airline enroute was foolish. The agent should have put up a red flag then and there. But US flight from SAN-PHX was late, (go figure). That is the only way I will ever travel US again, nonstop.
DL terminal in PHX is in T2 while US is in T4, hence, the PAX needs to take the shuttle bus for connecting. I guess, that was another complication.