Booking child for only first leg of the flight
Is it possible to book a child on their own PNR for only the first leg of a flight with their parents without it being considered an unaccompanied minor? In this scenario, my wife and I want to fly AUS->IAH->LGA and have our child fly with us from AUS->IAH where relatives will meet us at the airport to watch her while we are in New York. On the way back we would do LGA->IAH->AUS and go out of security at IAH to get her and bring her back on the IAH->AUS flight.
I attempted to call United but their only suggestion was to book all 3 of us for AUS->IAH->LGA and then tell the gate agent our child decided not to go to LGA. That didn't make sense and costs more than just booking our child for AUS->IAH.
Can I book a child ticket for AUS->IAH (roundtrip) if the parents are booked for the same flights but on a different PNR since we have the LGA leg. Also, she'll be staying with relatives so in no scenario would she try to get on the IAH->AUS leg if we were delayed in LGA for any reason.
Thanks