Without first discussing what the word magnitude means in mathematical context, it's really pointless to ask them this question. How can they think and learn about the magnitude of a rate-of-change if they don't know what magnitude means?
Maybe for 8th graders, you could just say that the magnitude of something is its size/force/power with any information about it's direction thrown away. Talk about speed being the magnitude of velocity. Where velocity can be negative or positive depending on whether you are going forwards or backwards (like a roller-coaster on its tracks), speed doesn't care about direction; it's just how fast the thing is going.