Restricting use of computer

Not sure if this is the right place to post this.

Hello I am setting up a desktop for my grandmother so that my little brother and sister can use it play online games, watch youtube, and do homework.

We don't want them to be able to install programs, change settings, use unauthorized programs,etc.

What is the best way to accomplish these things. The solution can be simile or advance. I am pretty good with computers and other technologies...

For example we don't want them installing toolbar, or any game that requires you download and install something.

Thanks!