There is not much dispute about what the Sun means, and it is worth saying so, because a domain full of contested readings can give the impression that nothing is settled.
The card reads as clarity, vitality, success that is visible to others, and the specific relief of no longer having to guess. Where the Moon before it describes a situation the querent cannot see clearly, this card describes one they can. That pairing is the most reliable thing about it.
Two refinements are worth making. The first is that the Sun is not a card of achievement so much as of exposure to daylight: what it promises is that the situation is now legible, not that it is now finished. The second is that its cheerfulness is sometimes uncomfortable in a spread. Total clarity is not always welcome, and readers occasionally find this card marking the moment a querent stopped being able to avoid something they had been managing quite well by not looking at it.
The child in both versions is doing real interpretive work. Whatever the card describes is being met without defence and without sophistication, and readers frequently take it as an instruction to stop being clever.
Waite's reversed meanings are unusually mild, the same happiness in lesser degree, a slight cloud, rather than a negation, which is another example of how far his reversals depart from the simple opposite-meaning convention taught in most modern beginner books. Anyone who has been told that a reversed card means the reverse of the card should spend an hour with Waite's own lists; the convention does not survive the encounter.