When my kids got to an age they started asking for YouTube videos, I found it entirely frustrating that Youtube Kids was just not feasible to setup like you describe. It was a choice between 3 things, everything, the "kids" section whatever that meant for YouTube, and just tell them sorry, not happening. Within the kids section there seemed to be some controls to help, but I found they did not work well at all and I could easily watch anything in the expanded kids section. I'd have been happy with a whitelist for channels.