We just watched it and I'd consider it inappropriate for a 9 year old. We enjoyed the movie, but thought it was too short (at slightly over 2 hours) to really reflect what's in the book. My wife read the book, I plan to read it this week. Even without reading the book, I could see how condensed it was. Probably should have been a trilogy such as The Lord of The Rings was.
Great cast, beautifully filmed but it seemed crammed into those two hours +. All in all, we enjoyed it, but I'm really surprised that someone would want to have a party with kids that young with that particular movie as the centerpiece of the party.
First, yes it was quite condensed, but the LOTR was made into three movies because it was based on three books and the movies were really condensed versions of their respective books.
Second, Hunger games is based on Book 1 of a trilogy. There will be three more movies (Book 2 and a movie split into two parts based on book 3)