It's best to call Verizon's customer support or go to one of their stores to get an answer to your question.
In my opinion it should be doable since you and your son / son-in-law are both in the Verizon network. The best thing is for the two of you to go to a Verizon store so that customer service can identify both of your accounts and your son / son-in-law accepts the transfer to his plan. Of course, if the two of you live some distance apart, then going to the same Verizon store is not convenient.