My wife was asked to throw away milk that she prepared in a sippy cup for our almost 3 year old autistic son from AA flight to YYZ-ORD . They didn't care her explaination and told her he is a big boy! I notice & experience the same thing for at TSA precheck domestic flights too. They say He is big boy and they ask us to throw away the water or milk. On rare occasion, they would test the drinks and let us carry them with us.

But when he was baby or younger they just would say they have to test, they did test and let us carry all the time.