If Verizon offers this service at no charge to you, who will pay for it? Me? The one with no kids/teens? To offer a service means that Verizon has more technology to build and maintain and that costs money. The cost has to be handed to someone. In this case it is offered as an additional service with a fee. You either subscribe to their fee based service or you parent as suggested in a previous post.
Now, if you want a free tracking service you do the following:
Lay out the rules to your children by letting them have a smartphone you must have their usernames and passwords. If they have iPhones you will need their Apple ID and PW. If they have an Android you need their Gmail email address and PW. To track an iPhone you go to icloud.com and enter the Apple ID and PW and you can locate. For Android you go to google.com/android/devicemanager and enter the Gmail information. Know that the device being tracked may receive an alert that it is being tracked.
However, please don't ask for an additional service to be included in the monthly plan for everyone when not everyone wants or needs it. I have enough fees attached to my bill I don't need any more. That is like me saying that I don't want to pay the $1 per month for Ring Back Tones and that it should be free. Do you want Ring Back Tones? Do you want to pay for something you don't want or need because I ant it to be free?