It's impossible in some areas to cover content without interjecting at least some level of opinion or bias. Furthermore, it's important to introduce some types of issues which are controversial. And, it's equally important to teach students debating skills.
What the hell are students supposed to debate? Which color to paint the hallways in the school? Or would you rather that students don't learn such skills?
My senior year of high school, I was in AP Government, and at a private Catholic school. We (the 18-20 in the class) suspected he leaned right on politics, but he never gave us any solid proof of it, or admitted anything, until the very last day. So it absolutely is possible to cover even controversial issues in unbiased informative methods.
For that matter, even if you do believe that opinions must be shown for some topics, fifth grade is too young for it. At that age, they're still likely to take the word of a teacher, or a parent, for an issue, and not really look into it themselves, or to understand it.