I don't mean to be nit-picky. Devise a scenario, Pin> who, what, when, where and how. Out in broad daylight, very quickly, no noise, find a way to get rid of the body. Plus, as a kid, you may be accountable to be somewhere else, plus if there was more than one, somebody might blab. I don't get the feeling that LE ever suspected kids, either. And whether I like what they've done or are doing, it is their area of expertise. As for me, I am getting to where I am posting in all caps so I am going to stop now, lol.
Kids might tease her (did it) or whack her or even beat her up, but I doubt they would kill her when her sister and brother might have doubled back looking for her while they were doing it. Whereas, an adult could have done it quickly and efficiently before the kids had a chance to know what happened.