Here's the changelog for YouTube Kids, which is really just a long sentence this time.
This little update includes an immersive full-screen view, voice search improvements, and a few bug fixes to make exploring and watching YouTube Kids even more fun.
With immersive mode, all the on-screen buttons and UI stuff should go away, which it probably should have done in the first place. I'm unsure if the voice search is actually better, but you can be the judge of that. Or your kids can, whichever.