To prevent access via safe mode you just need to set a password on the Administrator account. The Administrator Account that I am referring to is a hidden account that is installed when you install windows, it does not appear in your normal boot user list (it only appears when you boot into safe mode).
I just posted a guide on the main site a few days ago on how to change the Administrator Account Password:
http://www.fixmyxp.com/content/view/181/129/Follow the guide. The only additional steps you will need to take is after you have set the Administrator Password, in that same window will be the list of users, make sure to double click on any of the users that your kids use, then click on the Group Membership Tab, then make sure they are set as "Standard Users" (We don't want them set as "Administrators" because that would enable them to log in to Safe Mode with their own user account).