I lost the news report. Do you have a link? I saw the headline this am and immediately thought of this twisted little website - stickdeath.com. My kids think it's hilarious (and I watch what they do, but saw they lost interest quickly). It's basically stick people doing things that cause blood, like ripping another stick figure's head off. Twisted for sure, but something to arrest a child for imitating? No way.
Besides drawing is an expression, not an act. I would want to find out why they drew what they did. These kids were old enough to be skillful past the stick figure level so it was no accident that was the form they used, hence I really think it might have been a copy of the above website. However, asking questions to determine if their are upsetting issues in their lives is important. Kids do sometimes use art as an expression of what they are feeling.
However, boys will be boys. I don't allow guns in the house yet my son used to make them out of legos. I hope it truly pans out to be nothing, but would love to see the reason they felt a need to arrest children for drawing.