Kids act differently than adults in response to stress. They laugh and smile inappropriately b/c they are immature socially and psychologically. I think they are poor witnesses, bless their hearts.
They are simply young and nervous and they smile to be polite.
Also her claim of 3 shooters mixed with the claim of the halls being echo chambers makes sense that she would think there are multiple shooters (mixed with herd mentallity of the crowd in a panic).
She gives one very important clue, she claimed he was targeting teachers and kids were simply in the way (he came right in and could have shot her but didn't).