This app was mentioned in 6 comments, with an average of 2.17 upvotes
There are some kids zone apps. I'd have to find the one I used to use. Although be aware of the awful content on YouTube Kids. I don't let my daughter use it anymore.
https://play.google.com/store/apps/details?id=com.ootpapps.kids.zone.app.lock
This allows timers and app restrictions. It's basic but works fairly well.
That's really odd, even full screen games should show up in overview. Maybe try hitting overview right when the app begins to load, like when the screen is still black or on a splash screen.
Another option is this app, which might be overkill but seems to do what you need:
https://play.google.com/store/apps/details?id=com.ootpapps.kids.zone.app.lock
I don't know of a way to keep your kid in the one game you want to have her play, but there are many kid launchers that make a separate home screen just for your kid.
My favorite is this one
https://play.google.com/store/apps/details?id=com.ootpapps.kids.zone.app.lock
Alright so, if your cousin keeps downloading things you can use an app for kids games. That way they can't download anything without your access code, such as your phones pin number. This is one I like (link to Google play)
My daughter's four, she has her own tablet. It's one of my old ones. I use Kids Zone to restrict her to certain apps (more to keep her from going and poking around with the settings or making any in-app purchases than anything else). She's not allowed to have it in her room, only in common areas when we're there with her. She's got her own Netflix profile and a few age-appropriate games, and it keeps the living room TV available (and I don't have to watch Moana, again).
Some Samsung devices have a built in parental control app better than anything on the play store. It's the best Samsung feature. Can you upload some screenshots of the application manager from settings?
If there's no Samsung "Kid Zone" or "Parental Controls" this is an awesome restricted launcher: https://play.google.com/store/apps/details?id=com.ootpapps.kids.zone.app.lock with it you can stop users from accessing any app.
This app locker has worked well for me also, though I prefer the launcher approach: https://play.google.com/store/apps/details?id=com.domobile.applock
The problem is that if you allow the web browser, the user can get to anything on the web. I have never found a good browser with parent control. I suggest YouTube music or YouTube kids instead of YouTube. Good luck