That flight likely landed at one of the D60 gates, in which you deplane on a stand and then walk towards the terminal. Likely being a 12 year old the girl didn't wait for the FAs, the FAs may have been continuing on that a/c and with Eagle turns are very tight-30 minutes, so they had to immediately prepare for the next passengers. So the girl goes walking towards the terminal in which you enter a side door. That small area downstairs was never made for the volume of people. Even if the father was standing in the gate area they could have easily missed each other.