For once, I can actually sympathize with a "think of the children" argument, actually.
It comes down to choice. No matter what some of the more irredeemable parents seem to think, no one has to let their child play the latest FPS game, or have unrestricted access to the Internet. If something scary or disturbing comes on the television you don't think your child should see, you turn it off or you get them out of the room and occupy them so they don't come back in while someone is getting their pancreas pulled out through their nose. (And in this day and age, nearly everything on television comes with ample warning.) Even on a playground, you have the option of saying, "Hey, that looks like it might be a bit much for you, especially if you're playing on it like some of the big kids; why don't you come over here and play on this, instead."
It's called being a parent.
You don't really have that option with a billboard. I suppose you might go around it assuming you know it's there- but should you have to? Should you really have to choose between going a mile or more out of your way and giving your four-year-old nightmares? No, I don't think that's a reasonable standard.
It's not taking anything significant away from anyone to say that such a billboard shouldn't be on display; the show will still exist, and adults are welcome to watch it, and parents can decide for themselves about their children. In this particular case, insisting on keeping something in spite of others' discomfort isn't about maturity or artistic freedom, but selfishness.