Bad service from Etihad Airways at OR Tambo International airport

Etihad Airways staff at O. R Tambo international airport is the worst possible. They have no manager at work . My sister is traveling alone with 2 children, they have some hand luggage space that they would like to utilize but the children could not pick it up and the airline refused to book it at any cost. I don’t think it’s too much to ask if she’s a solo traveler and wants to use her luggage space that she paid for, also the children have bought full tickets and literally weigh less than 20 kilos which is way less than the average traveller. So I don’t understand why such arrangements could not be made.

Why do managers exist? If they can’t sort out these problems on the spot.

Also to mention that Etihad airline staff in another country she was traveling from did not give any trouble with regards to making this arrangement. They booked everything easily and also assisted her with going to the plane.