We have done ETWB many times. Every kid that is there can get in line for a photo, but they do not always announce it. As far as the PPs with kids, if they do not participate then when it is picture time, just send or take the kids to the line.
As far as OP, for a group of adults, I would not waste a FP on it. Usually during the day, the only "adult" roles are the guards and the wardrobe. The adults who play the guards get photos and the adult who plays the wardrobe sometimes gets a photo. Outside of that, not many adults get in line, and if it is busy, I don't know that they would let the adults get in line. For an adult who wants a pic, or even maybe to participate, what I would do instead is go at night. My DD was a night owl and we learned back when ETWB new and the lines were hours long, that if we went after 8, or even better at or after 9, in the evening the lines were significantly shorter. In fact, there was even a time that we had to wait for more people to show up as there were not enough. We have never had to wait more than 10 or 20 minutes for it later at night, even when it was brand new. Our friends do the same and in more recent times, they have had nights where the adults were doing some of the roles because there were not enough kids. When the lines are slower and it is not as full, I would think your chances of them allowing you in line for a picture are much much better. No reason to waste a FP. If you don't have to worry about kids who may be too tired to go late in the evening, you can just do that and usually avoid the long lines.
You can always go get a picture with Belle in her blue dress in EPCOT instead of doing ETWB. If her gold dress is really important, there is always a meal at Akershus in EPCOT.
