From what I've tried out so far (don't need to bother with RS children since I use it in my personal profile) Rustic Children is the only one I can in good conscience recommend for STEP currently. Skyrim Child Overhaul is in my opinion simply not mature enough and apart from the non-voiced deal the morphs need work, if it had more options and the bugs were worked out and more work put it it might be a contender ideally something allowing for the combining of the textures of something like Prince and Pauper or Rustic Children with Skyrim Child Overhaul would be great.
Well, it is a WIP and some of those issues are being taken care of as we speak. I'm going to hold off on a STEP recommendation for a bit to see how this develops. I've already put it in my personal mod list.