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.)