Hello there Calibabe86,
I understand you're looking for assistance with downloading an app on an iPad with parental control restrictions, so let's check into what may be preventing this specific download.
Let's check the parental control settings to determine if this is related to the notification that you're receiving. Let's make sure "Installing Apps" is set to allow with the password as opposed to not allowing it. For help with checking this, follow the steps within the section labeled "Prevent iTunes & App Store purchases" of the following resource:
Use parental controls on your child's iPhone, iPad, and iPod touch
If you see that apps are set to "Allow" for installing, let's see if this is isolated to Roblox only or see if this occurs with all app downloads that are attempted. If you see that this is only isolated to Roblox, it would be best to Contact Apple Support to look into this.
Have a great day!