I think bradk is on the right track.
The big question is, can you put your daughter on your room reservation? I'm talking in-name-only, I know she won't be staying with you. I (and bradk) ask this because she can't take DME unless she's named on the reservation.
Hopefully, you booked room-only and not a package, and hopefully your daughter wouldn't put you guys over the people-per-room limit.
If you booked a package, everyone on the reservation has to have the same entitlements. SO if you bought the
DDP, if you put your daughter on the reservation then you'd have to buy the DDP for her, too, and that would make it one very expensive DME ride!
If you are on a package but NOT buying the DDP, hopefully you are more than 45 days from your trip. As long as you are before that deadline, you can "downgrade" whatever tickets you ordered to one-day base tickets and then individually upgrade the tickets of the "real" people in your room after you arrive ... on second thought, that would be too expensive, too, as a one-day ticket for her would be in the neighborhood of $70, still more expensive than a taxi.
IF she would fit in your room, and IF you have a package reserved, and IF it's not past the 45 day mark, would you consider changing to a room-only reservation? That way, you COULD add her without adding any expense.
Let's say none of the above works ....
Mears will not take reservations for or sell advance tickets for a bus ride to a WDW owned-and-operated resort. But at the airport itself, you can walk up to the Mears counter and buy a ticket (one person on Mears would be cheaper than a taxi)., and it is quite likely she will be transported on a DME bus, anyway! The tricky part here ... and one that I have no knowledge of ... would be before you actually buy the ticket to find out if it can be arranged for her to be on YOUR bus. That would be the key.
Complicated enough???