I copied these comments from another site.
We should not despise or look down upon these little ones. We should treasure and greatly esteem them.
Jesus reveals a reason why we should not despise them. The reason He offers is because their angels in heaven continually see the face of My Father who is in heaven. Jesus says that these little ones are represented in heaven by their angels. Their angels are always before the face of Jesus’s Father and has His full attention. Nothing that happens to these little ones, including every way someone treats them goes unnoticed or unrequited by God. Therefore, see to it that you do not despise even one of these little ones.
Here it seems highly probable that Jesus refers to literal children having guardian angels, given the context of Jesus’s words. But it could also be that this principle applies to those of lesser station, which is a greatly comforting thought. This would mean that those who the world does not esteem, those who cannot repay us for rendering them service, have such great value that God assigns angels to watch over them. Keeping that in mind might help us change our perspective about who is truly valuable.