With Roku being one of the most sold streaming boxes and even on tv''s themselves, on the market today, I would think Roku has something to do with not having an app for YouTube Kids at this point. I believe they have enough clout to get Google Play Store to get an app for them as they have for android tv.
Yes, the YouTube Kids content filter isn't perfect, but you are letting perfect be the enemy of the good. You know what else isn't perfect... seat belts.
Amazon and Google have worked together to allow YouTube Kids on Amazon Fire devices