You can reach State Farm by dialing 1-800-STATE-FARM (800-782-8332).
Good news, though! In the future, when your children are of legal driving age and you are interested in adding them to your policy, you will not need to call a State Farm agent to deal with this matter.
While you cannot add another vehicle to your insurance policy online, you can request to add a driver online. An agent will receive your online request and process it for you. So convenient!