Which theorists developed a temperament classification system in which most children are considered to be easy, difficult, or slow-to-warm-up?

Chess and Thomas

Explanation:

Chess and Thomas were the psychologists who worked on Temperament inventory. They are both have proposed temperament inventory type:

In these types of temperament inventory Chess and Thomas discuss how much babies adjust. In easy-going babies, around 40% of babies easily adapt and adjust the new situation. On the other hand, a 15 % difficult child adjusts in the new situation. Only 10% slow to warm up the child adjust in the new situation.  

Thus there are different inventories given by a different psychologist.