The characters at the Dance Party tend to be a lot easier to get a picture with than at other small entertainment offerings such as the Hoedown. If there are too many kids, then you may be out of luck, but generally speaking we have always found that the characters are more than willing to pose - albeit briefly - for a photo.
The Dance Party happens several times per day at the Rockettower Plaza Stage right in front of Space Mountain. It may not always be featured on the park schedule, so it is best to check with City Hall on the day of your visit.