Just finished watching this with my 6 year-old son.
This was a pretty strange movie. As far as Peanuts movies go, it's definitely The Empire Strikes Back or Temple of Doom. There's violence, mistreatment and abuse throughout the movie. The whole thing is soaked in anguish and ignited with Lila's hideous manipulation of Snoopy. I would not be surprised if she was in the hospital because her mother was poisoning her in some sort of Munchausen-by-proxy affair.
Both she and Clara are psychos. Just because a person is singing a song does not mean they're not crazy and even dangerous. I started singing along with Clara (which helps my son to start singing along- he's shy about doing it) and when my son joined in I realized that we were mouthing the words of this crazy person. Like someone had put the Unibomber's manifesto to music- or that "Put the lotion in the basket" dude.
Anyway, I wouldn't recommend it. It's got more lows than a handful of quaaludes. The only upbeat moments in it seem to revolve around Snoopy's relationship with Woodstock.
One last thing: Didn't you find it strange that there were SO MANY TEARS coming out of Snoopy and Lila's eyes at the end? I really think they added twice, maybe THREE times as much because they knew there was little emotional investment in the characters and they hoped to trigger our emotions with a visual cue.