You will need to install a user script manager extension to install this script.
Under the Children's Online Privacy Protection Rule (COPPA) that FTC recently passed in US, YouTube had to make some changes on their website including disabling the miniplayer on content that is marked "for" kids".
This script bring back that functionality.
Note: This is my first userscript ever so it may be kinda inefficient. I got really annoyed by YouTube disabling the miniplayer on content that isn't even specifically made for kids (also watched by other ages) and took the matter to my own hands!
You are free to look at the source code and improve upon this script if you find a better way to do smth. I'm a beginner in JS so some things can definitely be done better :)