Mine would squeeze those tetra packs. Fun! Watch the juice go everywhere. Needless to say, they weren't too convenient for us but maybe I have evil children??

What's good about sippy cups is that you can have the Flight Attendants put whatever liquid you want in them. They fit well in seat pockets and can be saved for later.

The milk in tetra packs is a good idea for children who will drink it once, maybe twice. It quickly could get heavy, for such a long flight.