Jamieson, Fausset and Brown on Mark 9:36

36. And he took a child--"a little child" (Mt 18:2); but the word is the same in both places, as also in Lu 9:47.

and set him in the midst of them: and when he had taken him in his arms--This beautiful trait is mentioned by out Evangelist alone.

he said unto them--Here we must go to Matthew (Mt 18:3, 4) for the first of this answer: "Verily I say unto you, except ye be converted, and become as little children, ye shall not enter into the kingdom of Heaven:" that is, "Conversion must be thorough; not only must the heart be turned to God in general, and from earthly to heavenly things, but in particular, except ye be converted from that carnal ambition which still rankles within you, into that freedom from all such feelings which ye see in this child, ye have neither part nor lot in the kingdom at all; and he who in this feature has most of the child, is highest there." Whosoever, therefore, shall "humble himself as this little child, the same is greatest in the kingdom of heaven": "for he that is [willing to be] least among you all, the same shall be great" (Lu 9:48).