I had a similar problem with my 12 year old who wanted to pierce her belly button, nose and eyebrow. I told her "No Way!" She went and got it all done anyways.

I think this depends on the relationship you have with your kids. If I told my daughter "no" she wouldn't like it but she would listen. We have a close, warm relationship and I don't say no frequently. She respects me enough to listen on those few occasions that I feel I need to say no. I think that you're the Parent. If you don't want your 13 year old doing something - for any reason -  then you have the right to say "no" out of love. Sometimes the other kids really are doing things they shouldn't be doing. Why should you allow your daughter to join them? You're looking out for her welfare when she's still too young to take full responsibility. In a few years, you won't be able to do that anymore. So right now you have a right and maybe an obligation to lay down the law. If you're kid isn't going to listen to you, then you may have other problems going on - maybe the relationship has to be repaired.

you have to let your children have freedom because if you dont they will take advantage when they have a chance to get out and do things behind your back

You can't stop kids from doing what they want but you can punish them! I told my 15 year old if she came home with anything tatooted or pierced, she'd being staying home for 6 months - and she knows I mean what I say. I haven't seen any body markings yet!