Personally, I'm just reacting to the idea that it is some kind of simple open-and-shut court case. In Michigan what I'm reading is that excessive speed is not necessarily reckless driving (a felony in this case) in the legal sense, so that requires some argumentation about willful or wanton disregard as it is interpreted by the courts in Michigan (not by people here). Alternatively, we can try to prove a misdemeanor under 257.601d (the moving violation is the proximate cause of the death). I'd certainly agree that either change, lowering the rate of speed or running with lights and siren, lowers the probability of accidents of this type, but the standard of proof in criminal court is proof beyond a reasonable doubt.
To show that speed is the proximate cause, we're going to need an argument that doesn't permit the conclusion that he was traveling either too fast or too slowly. To do that, we need to work with the scenario where the deputy is in that same place at the same time but traveling at a lower rate of speed. So just imagine that the officer is doing 30mph and the child darts into his lane at the same angle. The officer has more time to react. Is it enough? Or if it still isn't, is the lower impact speed not likely to be fatal? To prove proximate causality, we need to prove beyond a reasonable doubt that the kid doesn't die in that scenario. The only thing that seems obvious to me is that the kid's odds are better, and that's not enough.
Theoretically, we could forget that and try to prove that the kid wouldn't have crossed at all if the officer was traveling at 30mph or had been running with lights and siren, but I don't know how we're going to prove that beyond a reasonable doubt, either. It's pure speculation.