It’s good to put things down in written form. We retain more of what we read than what we hear. It also makes things completely clear with no misunderstandings because it’s all there in black and white. This could be especially true with kids when you don’t know if they are just agreeing or are really hearing and understanding you.
Here is a free contract available for you to download. It lists 8 basic things they should do to practice online safety and has a place for signatures at the bottom. It would be a great thing to have posted right beside the computer.