EDITED TO ADD: Several other people posted while I was typing. I moved this phrase from the end to the beginning:
I've also heard the advice to go there at rope drop and you will have a short wait.
Keep in mind that this information about the waiting area will be outdated as soon as the renovations begin in Fantasyland/Toontown sometime in 2010.
We saw the Fairies twice on our trip in October. The first time, was during
MNSSHP and the wait was about 30 minutes, which I was surprised she did quite well for. There are fairy movies playing during the wait, which helped pass the time.
DD loved the interaction so much that she wanted to come back the next evening, during regular park hours. We were in the line that evening for about 45 minutes. That was much more difficult - the majority of people waiting seemed to have one or 2 of their party waiting in line and then, when they got up to the front, whichever adult was waiting used a cell phone to call the rest of their party to the front. This was hard for my DD since she constantly had people trying to squeeze past her wheelchair. Thinking we were getting to the front and then all of a sudden having an extra 20 people ahead of us was hard too. I thought of getting out of line and seeing if we could use DD's GAC, but by the time I thought of it, there were quite a few people behind us, so it would have been difficult to back her wheelchair out. Besides that, I was alone with DD, so I could not just leave her in the line while I went to check and the idea of starting all over with waiting someplace else was not appealing. As it turned out, DD wanted to see the fairies so much that she did OK in the line (but I waiting for her to hit someone who got too close or for her to start getting all OCD on me and took actions to keep her occupied.)
I would suggest showing the GAC to the CM at the entrance, expalin your situation and ask what else you can do. While GACs are not used for character interactions that are not in permanent locations, some people have used them for the permanent meet and greats, like the Princesses and the Fairies. It will not get you in sooner, but may give you a more appropriate place to wait.
There is an area with benches outside of the line where we saw families waiting until the member of their party who was in line called them. That seemed to be sort of regular operating procedure since we heard different CMs ask people at the front if their whole party was with them. Doing that might be an option for you, even without using the GAC and may be what the CM recommends if you do show your child's GAC.
The Fairy interaction itself was amazing. It was better on the MNSSHP, when they let people into the immediate waiting area in groups of 3 parties. The CMs letting people into the area were very good, in character and apologized for the long wait, by saying it was hard to get the fairies to settle down and pose for pictures because they loved to play.
Each party got a bit of time with each of the 3 fairies and each fairy 'walked' the group to the next fairy. By the end of the experience, each party had individual time with each fairy and a minute with all 3 fairies.
The fairies were playful and just 'right' and DD loved them. They did not bring in another set of parties until we were finished with the fairies. DD was listening to every word and they asked her lots of yes and no questions (she understands, but does not talk). They commented on the lights she had on her wheelchair and asked if Iridessa had helped her or if those were fireflies. They asked what kind of fairy DD was and I translated that she was a nature fairy, but really liked light and color too. One of the fairies commented to the other that DD was a very rare multi-talented fairy. The handler CMs were openly tearful and I saw a couple of the fairies blinking back tears. They were all so sweet.
THe next night when we went, it was much more of a conveyor belt. Since the wait was long and the park was closing soon, it was much more of a 'get them thru' situation.' They still let 3 parties at a time into the immediate waiting area, but there was a steady stream of people. It was much harder for DD because while she was with one fairy, there were groups of people with the both of the other fairies too (which was not the case at MNSSHP). She had a lot of trouble keeping focus and attention on what she was doing.
Our whole interaction in the fairy room was about 3 minutes the 2nd night. DD was still very happy, and the fairies were still wonderful, but it was not so magical. THe fairies both nights looked just right - I did let them know that DD had been there the night before (roll over the white space to see why -
the fairies were different the 2nd night). I said they might not recognize her because she was dressed differently and Rosetta said "You changed your petals. That's why I didn't recognize you right away").