For breastfeeding it is usually 2 taps to the opposite side of the chest (ie right hand pats upper left chest twice) Milk is the cow milking gesture. Or drink is the hand held in a "C" position like you are holding a cup and tilting it towards you.

DS currently signs finished and knows the sign for drink and more. They are such clever creatures to pick it up so quickly When he wants boobie though - signs aren't important to him. He just pulls the front of my shirt down and tries to help himself