Well, I believe that provided they are prepared for them, funerals should not be problems. Parents should explain what will be expected there, not just as far as behavior and manners go. For instance, if an open casket wake is expected, they should talk to them that a body in a coffin/casket on display looks like it's sleeping, but it cannot awaken because the person's body parts stopped working altogether.
Also, they would give them a choice after explaining. Should they not go, they should assure them not to feel ashamed of it.
As for dress, any neutral-colored, but not loud or bright (if the dress code for that funeral is relaxed as opposed to all black) Sunday best is appropriate. Boys may wear a dress shirt tucked in belted slacks, for instance.
I had attended 5 funerals throughout middle school, starting with my own grandfather's (I was 12.5 at the time) and wore black, sometimes with white. I was so prepared for these emotionally-taxing events because I drove my teachers batty by reading books about death and dying in elementary school from 3rd grade. I may be a weird and crazy kid, but the good thing about it was that it prepared me well.