You can always just hand it over and hope for the best....or go to one of the two ticket booths and they will do it for you, they do have change as well.
Try your luck, they rarely pay much attention to the ticket, and if they do stop you just say you've accidently picked up your sons ticket and you'll have to wait around for him to swap the tickets over. If thats the case, pop to the ticket collection, pay the £10 and upgrade.