Need a tablet locked out of all but a single app or How to load APK on Amazon Fire kids profile

I am trying to figure out a way to have several tablets in our manufacturing facility with an app that will allow workers to update data on work in process. The app is called "E2 SHOP Data Collection" from "Shoptech Software" on Google Play. There is an equivalent app for Apple, which I assume would be named similarly. We need to have all of our tablets set to only be able to access this app, or we will have a bunch of people goofing off in the shop playing games and/or pulling up all kinds of NSFW content.

I started with an Amazon Fire HD8 Kids Edition because it seemed like the perfect solution:
1 - It's Cheap
2 - The 2 year warranty covers it even if it gets physically broken. (always a possibility in our shop environment)
3 - The foam case seems rugged enough to help avoid said breakage.
4 - The Kids Profile is able to lock out all but exactly what you allow.
5 - Has a camera to support the bar code scanning feature in the app.
I was able to side load the Google Play Store and then this Data Collection application to the 'daddy profile', but getting anything transferred to the FreeTime or "kids" profile seems to be locked down tight if it's not an app from Amazon, and I have not found the back door to get it done. The write-up on howtogeek was awesome and detailed, but updates to the OS have blocked that method from working at some point since it was written in 2017.

Enough back story, here is the real question; does anyone have a solution to get this application onto the FreeTime Profile, or can suggest a particular tablet to buy that will accomplish the goals?