We always want our customers that are the Account Owners, have the ability to monitor their other lines on the account, Nannyboo. We are here to help.
Do accomplish what you are looking to do, you would need the phone itself for that line. Once you have the phone present with you, then you would register the line for its own My Verizon account online. Please be aware that if you are wanting to review the actual content of a text message, then the phone will also have to be enrolled in Integrated Messaging https://www.verizonwireless.com/support/verizon-messages-faqs/#subscribe-integrated
On your phone or computer you would need to access the account through the website, you cannot sign in on the My Verizon app on your phone. Also, if the user has an iPhone then any iMessages will not show, since those are not text messages processed by our network.