Quick answer
Character shows up early. The verse says you can read a young person by what they do, not by what they are able to say about themselves.
Read both lines
The word rendered "child" covers a wide range, from a small boy to a young man, so the verse is not making a claim about toddlers. What it is doing is extending to the young the same standard the book applies to everyone: conduct is the evidence. The second line supplies the two tests, pure and right, and neither is about talent or promise. Placed here it also does something quieter. Two verses after a question about whether anyone can call his own heart clean, the book says a person's actions are legible to observers, which is a different claim entirely and does not compete with it.
Related: Proverbs 20:9 · Proverbs 22:6 · Proverbs 22:15
New King James Version®, Copyright © 1982, Thomas Nelson. All rights reserved. Full copyright information. Scripture provided by API.Bible.
Go Deeper
If this verse has been used to put weight on you that it was never carrying, ask about it directly.