John Broadus (as does John MacArthur and other commentators) commenting on this verse believes that Jesus is referring to child-like believers or those young in the faith and is using actual children as His object lesson.
Broadus, “There is in this no sufficient warrant for the popular notion of “guardian angels,” one angel especially assigned to each individual; it is simply said of believers as a class that there are angels which are their angels; but there is nothing here or elsewhere to show that one angel has special charge of one believer.”
“It cannot be positively asserted that the idea of guardian angels is an error, but there is no Scripture which proves it true, and passages which merely might be understood that way do not suffice for the basis of a doctrine” (Broadus, Matthew, p. 385)
BUT, SCRIPTURE TEACHES THAT ANGELS MINISTER TO BELIEVERS AND THAT THEY DEFEND BELIEVERS.
Important Point: We’re never told how to discern this. They do minister to us. HOW specifically they do, we do not know – and GOD DOESN’T TELL US!