I really like my computer, but my brother spoils the fun when he downloads junk, such as Roblox, on my computer. I know there's probably an easy route that I've forgotten, but is there a way to make the user put a password everytime you install something? I've tried making the current user standard, but i only put the admin password for only a little number of stuff!
However, please keep it in mind that although Roblox "doesn't modify OS" per se, it (and other software downloads) may and often do try to sneak in other products. The subsequent installation process will attempt configuration changes. E.g., modifying your starting webpage. Or automatically launching some app at boot time.
If the downloader is not paying attention or just does not care then all sorts of junk will end up being installed.
Watch for those little "check" boxes that automatically opt you in for something or automatically opt you out of saying "no".