re: Umbrella policy / general liability

First , 16 year old kids can't own cars or anything else as they aren't of the age of majority in most every state.

Second, transferring title to another household member, even when all residents are of the age of majority, doesn't shift the liability, as all umbrella policies' coverage limits apply by statute to all related household members regardless of age. So your minor & adult children who live in the same home must be rated for by the insurer, as all household members are entitled to coverage regardless of title to any particular vehicle.