Jeanette,
I have a myspace page and can tell you that although you can set your account to private it will not keep your kids from seeing things they don't need influencing them.
Some of the good things - I originally created a page to keep up with friends/family that were on and we don't get to see often. It has allowed me to re-connect with people that I graduated with which has been wonderful.
Some of the bad things - For some people it's become a place to grip. For many kids/teens it's a place where they can get away with the foul language their parents would die if they knew about. You can send/receive emails that are not kept so parents don't have a way to monitor, this is not like Outlook where you could go through the computer to retreive messages and check on your children. You can search for other people's pages and like the internet when you search it will pull up people of "night-time" professions/unlike the internet you can't block the site & especially the photos that they have posted on their site. This is an easy way for kids to fall into pornography and have a way of communicating without parents knowing.
Although, I'm sure your children don't want to be involved or have anything to do with the negative that is on Myspace the site can not do enough to keep teens/children out of harms way. It's there for anyone to find.
It would be my recommendation that you do not allow it. I would also recommend that you try it out for yourself so that you are making a decision based on what you have seen and not just been told and heard.