I have done academic research on IB high school programs, which can be stressful for some high school students (as the high school student pointed out), especially if they had not been exposed to such rigor and theoretical thinking before. The IB program is philosophically different from standard education, even AP, as it takes a holistic approach to education. They develop critical thinking skills, give equal importance to all subjects (rather than emphasizing subjects that students already excel in, like AP or honors), and teach students to imagine themselves in their local community, in their own culture, but also as part of the broader world.
Personally, I love the philosophy, but it is not for everyone, just as homeschooling is not. Like someone else suggested, check out the IBO website, tour the potential school and try to meet the faculty, and see if it's a fit for you. It's great that you have the option to start your child in the program early (and if it is a fit, IB at the high school level will probably be much less stressful. Many kids talk about their IB friends and teachers as a "family," something that seems to get lost in very large, public high schools nowadays).
Whatever you choose, good luck! I have a first grader in a normal public elementary school, and we've had a good experience there, we just work on the critical thinking and global understanding at home a bit, to build those skills where the school does not.