This is from Minna no Nihongo Chapter 15. It's a description of Santa Claus.
みんな seems like a counter of sorts (I don't know how else to describe it) for 世界の子供. I always thought it was of the same grammatical category as 彼 and 彼女. So is みんな basically a "counter"? All the time? Most of the time? Depends on the situation?