Why is present progressive tense is used for "be playing"? Can't I just say "not normal for kinds to play outside this much"? What is the meaning of present progressive here?
Present progressive tense is used here because this tense emphasizes an action that is currently happening and continuing. Because the little boy is currently playing outside and is going to continue playing outside, he uses present progressive tense. You can also say "It's not normal for kids to play outside this much", but this sentence does not emphasize that the action of "playing" is currently happening and going to continue.