Set only ONE account to be the computer administrator. Password this account. DO NOT let your kids get on it and DO NOT tell them the password.

Give you children their own individual accounts or one account to share and make sure they are just users, NOT administrators.

And don't make the password something easy that they will guess. I remember when I was younger and I could guess my parents' AOL password very very easily and get on their account and unblock stuff when they tried blocking me from like AIM and stuff.