Technically, no. The Family Boarding policy only includes children up to age 6. But this is very dependent upon the ops agent. I suspect you won't have a problem, but you could run into an agent who makes you all wait until your assigned positions (assuming there's not another adult traveling who can wait separately with the 8 year old). Good luck!
Do families get to preboard?
An adult traveling with a child six years old or younger may board during Family Boarding, which occurs after the “A” group has boarded and before the “B” group begins boarding. If the child and the adult are both holding an “A” boarding pass, they should board in their assigned boarding position.