Rather than make a list of "classics of children's literature," the resources below discuss the idea of what makes a classic piece of literature in any field of literature and in children's literature. Sometimes, these lists of classics are called a canon.
From The Sage Dictionary of Cultural Studies entry for "canon":
"The idea of a canon refers to a body of work held to be the most important within a particular tradition, or sometimes to the parameters that surround the work associated with a given author. The concept derives from the Greek word kanon that means measure or rule and was further developed in the context of fourth-century Christian scholars concerned with the orthodox and the heretical. Thus, a canon is centrally involved with questions of inclusion and exclusion and in the context of contemporary culture that process of selection centres on questions of aesthetic value."
Chapter 7 of The Cambridge Companion to Children's Literature¹ (on reserve at the KCC Library) speaks of "Classics and Canons" of children's literature
Also, please see lists of award winning children's literature titles in that section of this research guide.
¹Stevenson, Deborah. “Classics and Canons.” The Cambridge Companion to Children's Literature, edited by M. O. Grenby and Andrea Immel, Cambridge University Press, Cambridge, 2009, pp. 108–124. Cambridge Companions to Literature.