If it is a computer in which ALL of you have to share... the kids NEED to realize this. It is NOT "theirs." Make that clear. Clear as crystal.
AND YOU... decide, what websites they can join. ONLY you... can make their accounts for it. And ONLY... children sites. There are many cute kid sites.
ie: Webkinz, club penguin, PBS, etc.
With my kids, they don't get too hung up on the computer. They can use it. They are reasonable and get off it when we tell them. However, they also use it for school, work. And for fun.
AND they ALWAYS, ask us first... if they can look at something. We do not allow them to just "google search" things. My 9 year old knows how of course and is computer savvy. BUT she always.... asks us first and she knows what is inappropriate.
AND in school, they have been taught that as well. Clearly. They have web/computer safety workshops as well as their regular computer class.
Even my 5 year old, is taught that.
To prevent your kids from just hopping on the computer whenever.... you make a password for it, to log on. AND do not tell them the password. That way, they have to ask you if they can use it.
We are a tech family and have lots of tech things in our house. But my kids never get hung up on it. And we don't treat it as a "forbidden fruit."
They have never gotten "addicted" to it.
They display responsibility about it. Even my 5 year old.