By A.E. Waite (1911), tarot card illustrations by Pamela Colman Smith (1911)
Material happiness, fortunate marriage, contentment.
Divinatory Meanings – Reversed
The same in a lesser sense.
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.
19. The Sun. The luminary is distinguished in older cards by chief rays that are waved and salient alternately and by secondary salient rays. It appears to shed its influence on earth not only by light and heat, but–like the moon–by drops of dew. Court de Gebelin termed these tears of gold and of pearl, just as he identified the lunar dew with the tears of Isis. Beneath the dog-star there is a wall suggesting an enclosure-as it might be, a walled garden-wherein are two children, either naked or lightly clothed, facing a water, and gambolling, or running hand in hand. Éliphas Lévi says that these are sometimes replaced by a spinner unwinding destinies, and otherwise by a much better symbol-a naked child mounted on a white horse and displaying a scarlet standard.
By S. L. MacGregor Mathers, c 1888
The Sun – Happiness, Content, Joy.
Divinatory Meanings – Reversed
These in a minor degree.
Symbolism of the Keys
The Sun – The Sun sending down his rays upon two children, who suggest the sign Gemini. (Behind them is a low wall.) It signifies Earthly Happiness.
General Book of the Tarot
By A. E. Thierens, 
Description and Meaning
“The walls indicate, that we are still in the visible, or material world.” (Papus) This relates to the picture which shows a child on horseback–or two children as in the older editions of the card–playing beneath the bright Sun and evidently within a walled enclosure. So far so good: we are and we remain in this world. And for the rest the Sun is the Sun and this card means everything that astrology can tell about the Sun, in every respect and on all planes. It means the positive or masculine elements in general, the power and function of will and concentration, great benefit and mighty protection in spiritual as well as in mundane life and matters. It may signify the father of the querent and high authorities, king, president, ruler, etc. The spiritual centre of man and the centre of importance in everything is indicated by it. Physically it indicates the heart and the solar-plexus.
The protecting power of the Sun is well illustrated by “the hieroglyphic value of the Hebrew letter Quoph, which expresses a sharp weapon, everything that is useful to man, that defends him and makes an effort for him.” (Papus).
In a figure laid for divination this card indicates the centre of interest and that which is fixed, certain, assured and under protection.
By Paul Foster Case (1933)
19, Le Soleil
Corresponds to the Sun, through the letter Resh. In its higher meanings it is a card of liberation, signifying release from the bondage of lower planes.
In material questions it is usually good, signifying gain, glory, or riches. In general it corresponds to the solar influence in astrology. But if surrounded by very evil cards it may mean vanity, arrogance, prodigality, ostentatious display, and a tendency to intrigue.
More information on The Sun Tarot card.