This mezuzah is posel because the yud in the word לבנך, l’vanecha (your sons, four lines down, first word on the line) is missing its leg. An impression of the leg is visible, but the ink from the leg has completely flaked away. It is not permitted to fix this because tefillin and mezuzos have to be written in order and this would be considered writing out of order. It would not help to show this letter to a child, as he or she would almost certainly call it a yud simply because of its small size. The general rule is that a child can only be asked in a case where there is a 50% chance that he will say one thing, and a 50% chance that he will say somethig else, like a short vov, in which case he or she really might call it a vov, and on the other hand, he might call it a yud. If it would be the small ‘kotz’ (literally thorn) on the left, bottom side of the yud, it would be repairable. In the picture below you can see the kotz shel yud, the small line on the bottom left side of the yud. A: the proper way to make the kotz. B: Kosher but not according to all the shitos (0pinions). C: Not kosher (but very common). D: Kotz according to some understandings of Rabbeinu Tam.