Remember that faster is a comparative adjective. While fast is used to describe things or people that are moving quickly, faster is used to indicate comparison. Thus, we can understand the baby as having used faster to indicate a speed that is faster than the current speed. He is using faster to urge them both to suck on their pacifiers faster.
Ex: The red car was faster than all the others. It was clearly in a league of its own.
Ex: Faster! You're driving too slowly.