Arthur E. Waite - The Pictorial Key to the Tarot :
The naked child mounted on a white horse and displaying a red standard has been mentioned already as the better symbolism connected with this card. It is the destiny of the Supernatural East and the great and holy light which goes before the endless procession of humanity, coming out from the walled garden of the sensitive life and passing on the journey home.
The card signifies, therefore, the transit from the manifest light of this world, represented by the glorious sun of earth, to the light of the world to come, which goes before aspiration and is typified by the heart of a child.
But the last allusion is again the key to a different form or aspect of the symbolism. The sun is that of consciousness in the spirit - the direct as the antithesis of the reflected light. The characteristic type of humanity has become a little child therein - a child in the sense of simplicity and innocence in the sense of wisdom.
In that simplicity, he bears the seal of Nature and of Art; in that innocence, he signifies the restored world. When the self-knowing spirit has dawned in the consciousness above the natural mind, that mind in its renewal leads forth the animal nature in a state of perfect conformity.
P. D. Ouspensky – The Symbolism Of The Таrot :
As soon as I perceived the Sun, I understood that It, Itself, is the expression of the Fiery Word and the sign of the Emperor.
The great luminary shone with an intense heat upon the large golden heads of sun-flowers.
And I saw a naked boy, whose head was wreathed with roses, galloping on a white horse and waving a bright-red banner.
I shut my eyes for a moment and when I opened them again I saw that each ray of the Sun is the sceptre of the Emperor and bears life. And I saw how under the concentration of these rays the mystic flowers of the waters open and receive the rays into themselves and how all Nature is constantly born from the union of two principles.