Chapter 18 of Matthew explains it pretty clearly that these little ones are not only referring to children, but to those who believe in Jesus and humbles himself as a little child.
2 He called a little child to him, and placed the child among them. 3
And he said: “Truly I tell you, unless you change and become like
little children, you will never enter the kingdom of heaven. 4
Therefore, whoever takes the lowly position of this child is the
greatest in the kingdom of heaven. 5 And whoever welcomes one such
child in my name welcomes me.
Jesus specifies who he is talking about in verse 6:
“If anyone causes one of these little ones—those who believe in
me—to stumble, it would be better for them to have a large millstone
hung around their neck and to be drowned in the depths of the sea.
We all have angels who watch over us, what Jesus is saying is be careful that you don't mistreat the little ones or the believer, and think of them as less, because God will know. And they are worth more than all the riches in the world to God, even though they may be worthless in human eyes.
Jesus came to die and save, even those whom we deem insignificant.
10 “Take heed that you do not despise one of these little ones, for I
say to you that in heaven their angels always see the face of My
Father who is in heaven. 11 For the Son of Man has come to save that
which was lost.[a]
12 “What do you think? If a man has a hundred sheep, and one of them
goes astray, does he not leave the ninety-nine and go to the mountains
to seek the one that is straying? 13 And if he should find it,
assuredly, I say to you, he rejoices more over that sheep than over
the ninety-nine that did not go astray. 14 Even so it is not the will
of your Father who is in heaven that one of these little ones should