Night of Joy itself is an after hours event in the Magic Kingdom, but they do bring crowds in. Also, as MKT said, the kids are particularly rowdy. One might think that since it's a christian event that the kids would be good christian kids. Generally not the case as these kids don't really have the best of behavior. Most of the groups are local and your trip should be okay.
I worked Night of Joy two years ago and the kids were not very well behaved. However, I also visited during Grad Nites and while some caused trouble, the trip itself was generally unaffected. The night of joy kids are high school kids, so from a guest's point of view, I'd imagine the day crowds are similar to those during grad nites.