Multiple iphones on one shared itunes account.

I have an Iphone and itunes account. My children also have iphones which are listed under my account. I'm having a problem when we use icloud, where all our contacts are getting added to each others phone and if I try to delete a contact, it deletes it off their phone. Is there any way to separate this and have it not happen. We obviously all have different phone numbers. So, why is all our info being put on each others phone and how do I stop it?????