Please note once again that I am not an attorney, and neither do I play one on television, and that nothing I say here should be construed as "legal advice."
Really, intellectual property infringement is not confined to edible printing, nor to copyright: assume your skills with a piping bag far exceed my own, that you can draw pictures with a piping bag and BC that look like they were drawn with pen and ink. Now suppose that instead of edible printing, you pipe Mickey Mouse onto a cake, in a situation that has never appeared in any licensed Disney product (say, plugging Mickey, Minnie, Goofy, and Donald into Edward Hopper's masterpiece of 20th century American painting, Nighthawks. Certainly plenty of artists have done variations on that iconic image, varying from Gottfried Helnwein's Boulevard of Broken Dreams, to a Christmas card with Santa and a few reindeer stopping for coffee, but I don't think anybody's ever done a Disney version) So long as the characters were recognizable, anything about your opus that would make it more difficult for Disney to get you for copyright infringement would also make it easier for them to get you for trademark misuse.
As to the matter of a customer-provided, customer-taken photograph of his or her kid with a Disney character, taken in a Disney theme park, well, you have a matter of intention, the matter of actual harm to the owner of the copyright and/or trademark, and of what the actual intellectual property is. Simply put, Mickey Mouse is a Toon. So is Bugs Bunny; so are Beetle Bailey, Smurfette, Edda Burber (from Brooke McEldowney's 9 Chickweed Lane), and Candy Moatmonster (from Bill Holbrook's On the Fastrack). They are all drawn cartoon characters, from either animated cartoons, or comic strips, or both. When you photograph your child with Mickey, at Disneyland, you're not actually photographing Mickey Mouse; you're photographing a Disney cast member in a Mickey Mouse suit (at least as of the first time I took the Backstage Magic tour at WDW, the officially approved technical term was to say the cast member was "assisting in the portrayal of Mickey Mouse"). And if you're taking pictures of your kid with Mickey, there's no real intent to harm Disney, or to profit wrongfully from their property, and (given as how no stock agency would touch such a picture with a ten-foot-pole) no actual potential to do either. Moreover, Disney management has always tolerated, and even encouraged guests to take pictures in most onstage areas, and tolerates guests posting their own Disneyland pictures on web sites. I would argue that in a "kid with Mickey" picture, the kid is the primary subject, and the character merely establishes place and circumstances, and the picture was taken in a place that was open to the public, and did not restrict non-commercial photography and that in printing the customer's photograph onto edible media, and mounting it on a cake, you are no more guilty of infringement than a camera shop would be for delivering the picture on T-shirt or a coffee cup.
Still, even as an amateur, I've been sticking with images where my rights are entirely clear: the speed limit sign on my parents' 55th anniversary cake was either PD, or it was CC or GPDL with "remix." The Leland Award image, and the International Printing Museum logo, were both available for my use because I was acting in my official capacity as a docent of that museum. The Wikimedia Commons images of Squad 51 and Engine 51 I've slated for my 55th birthday cake were both placed in the PD by the photographer, and any additional images going on the cake will be entirely my own work. And in the 10-minute DVD I recently finished putting together, of the Space Shuttle Endeavour's arrival in Los Angeles, was made from my own photographs, my own 3-minute video, a dozen or so official NASA photographs (PD by definition), and several photographs from Wikimedia Commons and Flickr, all of which were CC with remix, and everything not my own was meticulously credited.