I haven't been there yet, but as I understand it, Pixie Hollow is presently based on the premise that Guests "shrink down to pixie size" when walking into it, so that they are temporarily the same size as Tink & friends. Hence photos show same-size Ps & Gs.
I've sent suggestions to Disneyland for two other techniques, either of which could either replace or just be an additional option:
(1) In the recent films The Lord of the Rings, full-size actors appeared to be Hobbit-size by the construction of special sets which were built with a carefully distorted depth portion. Thus in Frodo's home Gandalf stood, sat, and walked in the front section, while Frodo was in the back section. To the camera's eye, the two actors appeared to be in the same room, talking directly to one another, with Frodo half the size of Gandalf. A similar device was used to show a "half-sized" Frodo apparently sitting "beside" Gandalf on the seat of a horse-cart.
One or more similarly-fashioned photographic sets in PH would result in full-size Gs standing together with a reduced-sized P. This wouldn't get the Ps down to Peter Pan size, of course, but it would still be dramatic.
(2) Another option would be to have one or more special sets where your personal P poses, while you the G pose in another. The two photos are then merged into a final composite, which is what the G can purchase. In this merger the P is indeed reduced to Peter Pan size, and appears standing in the palm of your hand, or perhaps sitting on your shoulder, or hovering in the air beside your head. It is of course the same P you met live in PH, not a standardized actor or cartoon,smiling righ back at you. The idea is that while PH magically shrinks humans down to P-size for visits, cameras are not so affected, and so photographs show everyone in their normal size.
This kind of composite photography is not all that difficult. At Siegfried & Roy's Enchanted Garden at the Mirage in Vegas, for instance, you can get a photo taken against a blue background. When you pick up the print, you're standing in a lush jungle surrounded by friendly lions and tigers.
Option #2 also offers some fun possibilities. Tink [and her fellow Ps] is famously not all sweetness-and-light. So:
(a) A G stands with an outstretched arm and pinched fingers. When he picks up his photo, he's holding Captain Hook's brass lantern, with an angry Tink [or other P] sitting inside.
(b) If you are a gentleman visiting PH with your "Wendy-Bird", you're standing there beaming at the camera, while your lady has a less-pleasant expression on her face, because a strand of her hair is being tugged by a pouty, jealous Tink flying by her.
... and etc.