PG-13 (PG-12) is really the new PG nowadays.
When there was no buffer between PG and R (back before 1984), PG used to be able to encompass some pretty heavy content. But now, the shift has gone something like this:
PG = Upper level G
PG-13 = PG
Look at a movie like "Raiders of the Lost Ark". Back in 1981, that movie was rated PG, and it had some pretty graphic content and language. I can guarantee that if that movie were made today with the exact same production values, it would've gotten a PG-13. Same thing with Star Trek II: The Wrath of Khan.
Thus far, the PG-13 Harry Potter movies have not been that bad in terms of content that might be too intense for young'n's. Then again, I don't have any kids, so I'm going strictly off the notion that not much fazes me in a movie.