“Love children especially, for like the angels they too are sinless, and they live to soften and purify our hearts, and, as it were, to guide us. Woe to him who offends a child.”

Book VI, chapter 3: "Conversations and Exhortations of Father Zossima; Of Prayer, of Love, and of Contact with other Worlds" (translated by Constance Garnett)
The Brothers Karamazov (1879–1880)
Context: Brothers, have no fear of men's sin. Love a man even in his sin, for that is the semblance of Divine Love and is the highest love on earth. Love all God's creation, the whole of it and every grain of sand in it. Love every leaf, every ray of God's light. Love the animals, love the plants, love everything. If you love everything, you will perceive the divine mystery in things. Once you have perceived it, you will begin to comprehend it better every day, and you will come at last to love the world with an all-embracing love. Love the animals: God has given them the rudiments of thought and untroubled joy. So do not trouble it, do not harass them, do not deprive them of their joy, do not go against God's intent. Man, do not exhale yourself above the animals: they are without sin, while you in your majesty defile the earth by your appearance on it, and you leave the traces of your defilement behind you — alas, this is true of almost every one of us! Love children especially, for like the angels they too are sinless, and they live to soften and purify our hearts, and, as it were, to guide us. Woe to him who offends a child.
My young brother asked even the birds to forgive him. It may sound absurd, but it is right none the less, for everything, like the ocean, flows and enters into contact with everything else: touch one place, and you set up a movement at the other end of the world. It may be senseless to beg forgiveness of the birds, but, then, it would be easier for the birds, and for the child, and for every animal if you were yourself more pleasant than you are now. Everything is like an ocean, I tell you. Then you would pray to the birds, too, consumed by a universal love, as though in ecstasy, and ask that they, too, should forgive your sin. Treasure this ecstasy, however absurd people may think it.