How to get the documents I need signed securely sent?

I need to send secured documents to parents to sign and I don't know how to do that. I sent out a document to sign, and the parent received a link to click to sign the document. The problem was that if someone opens that person's email and click on that link, they would have access to her private information. How do I prevent something like that from happening? I tried to put a view password but I could not have the document e-signed afterward. Help!