"YouTube Kids" is cleaner than regular YouTube. Sadly, I haven't found any method of disabling autoplay. If it's possible, how can I disable autoplay?
I checked on iOS 12.4, YouTube kids app, there's no feature for autoplay settings. It is "on", on web and on app.  Alternatively, I am not very experienced in user scripts, but can suggest a potential, easy algorithm. 

When the current video time = (total video time - 1) seconds, pause video. 

I have no idea how to write it, but you can get help from sites like 


https://greasyfork.org/en/scripts/by-site/youtube.com?site=youtube.com&amp;q=autoplay 
Github etc.


Do note that some Safari versions might resist installing user scripts, but if you have uBlock origin installed, your version should be good to go.