[Youtube app allowed while parental block for website is in place]

I have blocked youtube as a website on all kids' devices. However, on the android phone I cannot delete the app. Upon testing, it was easy to enable the youtube app, go incognito and access porn. This is not okay. Please help me to find a way to block all apps that cannot be uninstalled. I want to stay with Republic wireless and allow my kids access to some apps with basic talk & text options without the concern of the internet in my kids' back pockets.