In this question, i talk about if a Sefer Torah might have a misformed letter, a child who can recognize letters but not read is brought to say what letter it is.
If everything surrounding is covered though, why can't an adult, or child who can read, be brought? As long as they have no expectations for what letter it should be, what's the difference?
Why must it specifically be a child?
That's why The Shulchan Aruch Harav (32:20) writes:
ומכאן אתה למד שאין צריך לכסות מלפניו שאר האותיות כמו שנוהגין שא"כ אף החכם שמבין הענין שקורא מהיכן יבין לקרות אות זו כהלכתה כיון שאינו יודע ענינה ופירושה
ומכל מקום אות שנפסקה בנקב ונשתייר ממנה גם למטה מהנקב צריך לכסות מלפניו מה שלמטה מהנקב לפי שהוא יצרפנה עם מה שלמעלה מהנקב ויקרא האות כהלכתה ובאמת אינן מצטרפין כיון שהנקב מפסיק ביניהם
From here we learn that one doesn't have to cover the other letters as is the custom since then even if we bring a wise [child, who knows how to read] from where will he know how to read this letter properly as he doesn't know the context and explanation.
Nonetheless, one must cover the bottom part of the letter which has a hole since the child will naturally combine it with what's above and will read it fine, even though one can't combine them due to the hole.
Mishnah Brurah on O.C. 143:4 comm. 25 uses the term "tinok" (which often means a "baby", but, here I think he means a "young child") who is neither smart nor stupid (i.e. - he's not brilliant nor is he an imbecile) Is called to view certain types of mistakes such as erased letters or words that look like they might be connected when they should be separate.
Since he describes the type of child as a "mid-intelligence", I assume the reason is that we don't want someone who is even somewhat knowledgable of the verse he's looking at, which most adults are (i.e. - those that can read Hebrew and who are even minimally following the Torah reading. They know what to expect.) I.e. - we want a child who can minimally recognize letters, but has no bias as to what the word or letter should be. Most young kids aren't as well versed in Chumash to know what to expect. (By "tinok", I am assuming M.B. means someone below average "chinuch" age, say a 3-5 year old. Not sure, but that's my best educated guess on this.)
