Jesus extends the idea of greatness from being unconcerned about status and position to humbly receiving and welcoming “little ones” of all kinds. This refers beyond children to any weak, marginal, powerless, or poor person. True greatness is being open to engage the people at the bottom of the social ladder—those who are little people in the world’s estimation.
“...those whom God favors are granted grace not for any worthiness of their own, but by God’s unmerited mercy as a gift to be converted into living on behalf of others.” (Andrew DelBanco, Professor of Humanities at Columbia University, New York, in "A Smug Education," article in New York Times.