Determining form using a child
1. If one cannot tell whether a letter has the correct proportions, or is doubtful whether it has its form, and thinks it may be pasul, he shows it to a child who is neither particularly smart nor particularly backward, and if he reads it correctly, it is valid. Using a child to test a letter is applicable only when it is unclear whether the letter is valid; if we can see that the letter is not valid (and if the yuds of aleph, shin or ayin or the leg of tav had become detached, and so on), the child’s interpretation doesn’t help because we have seen for ourselves that the letter is not correctly made and is invalid.
2. A child who is neither particularly smart nor particularly backward is one who does not comprehend the context but knows and recognises the letters reliably. One need not hide the letters which follow the letter in question, but it is the custom to hide the preceding letters.
3. If a letter became broken by a hole such that it wasn’t clear whether the remaining part was of sufficient size to be valid, so a child had to be asked, and some of the letter remained below the hole, one must cover the part which remains below the hole, because otherwise the child will read that part along with the rest of the letter, and [his judgement will be impaired because] the parts may not be joined (Mishnah Berurah 32:10). Similarly, if ink had flaked away from part of the letter leaving its imprint, when one shows the non-flaked part to a child to determine whether enough for a letter remains, one must cover the imprint so that the child will not read it with the letter (Peri Megadim).
4. If the children don’t agree, we follow the majority (Le-David Emet).