tractable

If your little brother quietly obeys your instructions and waits for you at the food court while you and your friends wander around the mall, he's probably a tractable child, meaning he's obedient, flexible, and responds well to directions.

Note the similarity between tractable and tractor. Both come from the Latin word tractare, which originally meant "to drag about." You can think of a tractable person as someone who can be dragged about easily, like a plow being dragged by a tractor.