The major things have already been addressed in previous posts: aperture too small -> background too clear and distracting, flash too direct and too bright -> faces flat (and therefore boring), and with regards to composition: crooked horizon lines.
I feel many of these also further lack in the area of composition. Often parts of the children's limbs are cut off and in places that look random and like happens with many a snapshot shooter and their pics. From what I have learned right here on TPF joints should be a taboo for cropping into persons' limbs.
In the boy's pic (2) you can see that "fill-the-frame-compositions" do work with people-photography, though, so there is no saying that at all times the whole person needs to be within the frame (where would portraits go, eh?

). Also the funny one of the two girls pulling faces at each other (5) shows that of course (!) zooming in on only parts of a person is (very) permissible.
That one is my personal favourite of the series, by the way, and it may be the "lack of" background which helped it become my favourite. But it's also the funniest, and expressions like this are the most fleeting, which - to my mind - enhances their appeal (not to the teenaged girls, I'm afraid [!!!!!!!], but to most others

).
The last (which you also put into Critique) has so many artefacts across her face, I really wonder what brought those about?????????????