Except during special promotions run by ticket outlets, tickets for 3 or fewer days cost the same or only slightly less when purchased in advance compared with at the gate.
If the one day timeshare tickets can be upgraded (are standard magic your way tickets) it is cheaper to add days #2 and #3 slightly after use compared with throwing them away after use and buying more tickets. If the tickets did not come with hopping, the savings by upgrading as opposed to buying more tickets is only about ten dollars per ticket.
Some one day tickets from timeshare promotions are not standard and cannot be upgraded and cannot have hopping added. Some can be upgraded but from outside the park (more correctly prior to use) the starting value is lower and therefore the amount of money you have to add for more days is greater. You cannot tell until after you get to Disney. Ask the cost at Guest Relations outside before doing it. Get an individual price for each ticket, not jsut a total for a bunch of adult and child tickets. If you don't like the numbers, write down the numbers anyway, take back the old tickets unaltered, step out of line, and think about it.
On non-upgradable timeshare tickets, provided you can live with the restrictions, if any, and such as non-hopping, it is probably better for one adult to use both of them plus a new one day ticket on successive days and the others to get new 3 day tickets.
The 13yo can keep using any old ticket he possessed, or it will be exchanged free for an adult ticket if the turnstile CM catches him.
Some timeshare tickets have hard expiration dates even if not used, and this may limit your flexibility in saving them for a future vacation. OT: Because of this, doing a side trip to a timeshare to get tickets is not a good idea if your vacation is longer than 3 days and where using up a separate ticket for the fourth and succeeding days clips very little from your ticket budget.
Ph.D. in logic needed to understand what follows.
If the two timeshare tickets can be upgraded, prior to use the trade in value may be less than gate price depending on how much the wholesaler or
travel agent or timeshare office paid to buy them in bulk. The dollar number you get from the guest relations window outside the park will reflect this.
If the timeshare tickets can be upgraded, after usage (and within 14 days of first usage) their value for upgrade purposes equals the gate price of an identical ticket. But from Guest Relations inside the park should you find out that the timeshare ticket could not be upgraded, both of them have already been used by two different family members so you cannot take advantage of one person's using both of them on successive days to make possible a cheaper 3 day ticket for the other person.
Or if you decided first to let one person use both of the timeshare tickets on successive days, should you find out from inside the park that the tickets could be upgraded, nobody should use the second timeshare ticket because everybody else already got new tickets and it is cheaper for one ticket per person to cover the whole vacation.
This last paragraph needs both a Ph.D. in logic and a Ph.D. in mathematics to understand. Should you ask for and obtain a trade in value from Guest Relations outside the park (this implies upgradability), choose to use the tickets as-is, and choose to do the upgrade inside for the reasons in the previous paragraphs, then in the event the CM inside says you cannot upgrade the tickets, then Disney has given you a bum steer and owes you some extra pixie dust if your calculations and subsequent choice of who uses what ticket turns out to be wrong as a result. You would need to write a report using the numbers you wrote down at the Guest Relations booth outside to figure out what your ticket budget would have been had you known that the tickets were not upgradable prior to entering the park.
Therefore a catch-22 exists.
Disney hints:
http://members.aol.com/ajaynejr/disney.htm