Go buy one. Don't even hesitate. Especially if you're taking a Disney trip.
We have a monkey backpack/harness, as well as a generic blue harness. We took both on this trip, but my son liked the monkey one best. Whenever we'd take him out of the stroller for any reason, that monkey would go on and he got to a point where he quickly learned if he let us put the monkey on, he could get down and walk a little provided we felt it was safe for him.
Also, if you've never used a harness on your DD, start out slow. We actually started putting the monkey on my son 2 months before our trip to get him used to how it felt on his back. Then after a month of just wearing the monkey, we added the strap used by the adult to hold on to them, again, to get him used to how far he could go before he'd be tugging. The monkey did bug my son at first when we started making him wear it around the house, but after a few days, he was used to it and would wear it all day and not even realise it was there.
Don't think twice about not getting one, or not using one because your daughter won't like it. They are a life saver. Proof of that is at the airport, we put the monkey through the xray machine first, mom would go through with my son, then she'd grab the monkey and put it back on him while the other 3 of us would gather shoes, bags, and jackets. In fact a TSA agent at Orlando airport on Saturday commented on how organized we were while mom strapped Keegan back into the monkey while she waited for the stroller. The agent said that a few days prior, a little boy had taken off and run away from his parents and it took them quite a while to find him. She said that if the parents had had a harness, the situation wouldn't have been so scary.