I just want to piggy back on this. I have much the same situation. I am the administrator and I have 4 kids set up. I have one as locked down as I can get. He isn't supposed to have access to any browser, other than Edge.
I did the same thing. To 'test' the security I went to Chrome and downloaded it. I was then asked for an admin PIN. I clicked No but immediately got a 'you can download Chrome without an admin password' button and voila, I was able to download (and then use) Chrome. Which, completely defeats trying to lock down a tech savvy curious kid.
If I download Firefox or Chrome for myself on the Admin account, it then shows up on his login page. Occasionally, the Family settings blocks Chrome access, but it doesn't block Firefox access.
Help! I really like the features to lock out specific websites and that works great...when there is only access to Edge.
Glen