迷子 is read as まいご, despite the on-reading of 迷 being めい (for example in 迷惑（めいわく） ).
Why is that the case?
I would assume that this is not a case of jukujikun.
What I do find interesting is that the kun-reading of 迷 is まよ (as in 迷う), starting with ま, so perhaps まい is a combination of めい and まよ? Though I have never seen that before.
Or maybe it comes from the い-stem 迷う 迷い? Though that would require both dropping the よ and irregularly attributing the い to the kanji 迷, which seems odd to me.