lol, If you thought taking a 6 year old to a PG movie was a good idea then it is probably your own fault.
PG is one of those swing ratings, it used to mean everything that wasn't an R until Gremlins and Raiders of the Lost Ark came out.
I'd say any kid around 10 years old or so should be ready to see a PG movie without having troubles, but clearly that kid wasn't ready to see it. There are plenty of movies perfect for little kids these days that aren't like sitting through Barney the dinosaur for adults.
Tales of Despereaux is probably in that phase, or Bolt.
Sure, they may not be something that you'd go and watch by yourself, but you're toting a toddler around, you sort of have to go by their sensibilities... Did the kid watch the first Narnia? It wasn't as gruesome, but did they watch it and enjoy it and understand it? It had some pretty scary parts in it for little kids.
I don't think the mom was right in saying you should have watched the movie first, but I do think you should have gone to a kiddie movie with a kiddie.. They aren't all horrible for adults these days