He couldn't be charged with vehicular homicide because there wasn't any evidence that anything he did caused the collision. A "child dart-out" case is not a basis for a VH charge. The fact that he had prior DUIs and may have had a drink or taken pain meds at some time during the day doesn't prove (1) he was DUI at the time or (2) if he was DUI, that it made any difference in this case.
Leaving the scene means, of course, that there was no opportunity to give him a breath or blood test or note any signs of impairment. Which is why that's a criminal charge. But since that didn't cause the child's death, it's irrelevant for purposes of a VH charge. We'll stipulate that he's a lowlife, but that's not a basis for prosecution . . .yet.
See 51,