Flying with an 18 month old in a week?

My mother in law and I are flying to Oklahoma with my 18 month old daughter to see my husband graduate BCT in 7 days....She has told me that my daughter has to sit in my lap the whole flight?? It's 3 hours....problem is, she does NOT like to sit still, is that true? It's American Airlines? Also can you bring things like toys,portable DVD players to keep her entertained if that is the fact? Anybody with any advice on flying with a child that age, please help!