OK steve, the easier thing to do is to change the password on your account, and set up a different account for the kids.
When you go to your control panel, you shd see an option for user accounts and family safety.
First of all use the option to change your own password. Change it to something only you know. Treat your password like your toothbrush - don't let the kids have a go of it! Use something only you know as a prompt to memorizing your password.
Then when you've done that, go back to the screen that said 'user accounts and family safety'
This time choose 'add or remove user accounts'
Create an account for the grandkids - invent a simple password like 'kids2015' for them.
Then there's no way they can get into your stuff.