Definately try for a picture holding Tink from the Photopass people on MainStreet and in front of the castle.
As for when she meets Tink, it should be fine. The few times we have done it, the CM who walked us back to the room did a really good job of explaining that we were being "sprinkeled with Pixie Dust" as we walked down the hall and to notice that we were getting much smaller.
As you approach the door in your small group the trees will appear HUGE and there are lady bugs and mushrooms that are about your size. When you get in with Tink and her friends there are more things to help you feel small - giant teacups, stacks of pennies and a thimble or two.
If you end up having a CM that doesn't explain things, make sure to point it out to her. There are twinkling lights that look like pixie dust as you walk down the hall to help with the magic. You will also hear giggles and an occassional bell. If she watches the Tinkerbell movies at all then you can point those out as well and she will know what they are for.
So, just explain to your GD that she won't be able to hold Tink in her hand when they meet because she and Tink will be the same size thanks to Disney Magic and Pixie Dust! It worked for my 6 and 4 year old and we had a great visit. Those fairies sure do talk A LOT!!
Have a wonderful trip!
I can also PM you some pictures of my kids meeting Tink if you would like so that you can show her what to expect.