On our last trip (Dec 2015), my son and daughter did not participate in the show and did NOT get to have a photo taken with Belle. We were in total shock when we realized Belle had already left the room and we hadn't been called up from our seats for our photo. When I asked a CM, I was told that they have to be in the show OR join the parade that takes place very quickly at the end of the show. This was a change from previous trips where anyone who wanted a photo was able to have one taken, even if you weren't picked to be in the show. My kids are still pretty little and they were too timid to join the parade on their own, but in the future, I will plan on going with them so that we don't miss out.
From the time it opened, it has always been hit and miss depending on the CMs who are in the room as to whether they announce that kids who did not participate, but want to see Belle can get in line. They are almost always allowed to do so, but often the CMs don't actually announce that they are. (I've never seen them turn any kid away that got in line, but I have heard a story or 2 about it possibly happening). So sorry your kids missed it. We have done it several times because it is the only way to get a picture of Belle in her gold dress anywhere at WDW unless you eat at Akershus- and DD loves the princesses. When we were in there once, I saw 2 little girls crying because other kids were getting their picture with Belle, so I motioned to a CM and pointed them out, and she immediately came and got them and took them to the line. Another time, there was a little girl crying because she had not participated in the show as she was shy, and she really wanted her picture with Belle and to see Belle, but no way was she getting up and standing in the front of the room with everyone still on the benches watching as an audience. That time a CM brought Belle over to see her and speak to her on the way out of the room, but she did not get a picture with Belle.
The cottage and special effects are amazing, but for us (including DD) the show was cute the first couple of times, but then became just a necessary annoying thing to have to sit through for DD to get to say hello to and get her picture with Belle. We are not fans--- we really think that Belle should be available somewhere else in her gold dress (any park- but not requiring an ADR/meal) for a real meet and greet. Just a warning, unless you are positioned just right at the benches where you can get an angle to get a shot of Belle and your child, you will not be able to get photos with your own camera. I have seen a couple of parents not right there, but close to that area be able to jump up and get a shot, but we have also been there when everyone was instructed to stay seated in place by the CMs. They rush the kids through really fast for pictures. It is a few words from Belle and a quick snap and on to the next kid. Sometimes they turn out cute despite the rush, but we have had a couple of times where the photo was bad/blurred. We even had one time where they had some type of a glitch and lost all of the photos from our session and could not retrieve them (felt sorry for the parents of kids that had the bigger parts- especially the beast). If your child is one of the bigger parts, there will be photopass shots taken during the show. The real annoying thing is that they are all on one photopass number, so that when you put it in you will get about 100 photos added to your account, of which there might be just 1 of your child if they were not a part. Then you have to spend forever trying to delete all of the others out of your account.
It is definitely something worth doing at least once for a 5 year old girl, and I would not worry about a younger one being disruptive. He likely won't be the only toddler there.