Great question as I also have an 8 yr old I coach, and the fact that you were introduced as a "defensice coordinator" is probably more as a title to show you are involved with the team more than anything.
The dads I coach with, rather much more have some title and show they are involved in the team somehow other than just being a parent in the stands.
Take it as a compliment that addressed you as the defensive coordinator, but take it more as a justice to be there for the kids and being a part of the team.
On our team, we even gave one of the dads the title as "Rotation Director", when we ask certain players to sub in and out- he grabs them from the bench and helps roatate them in and out, but like I said, its more to show as he is helping and being involved with the kids. Take it with a smile as helping the kids and being involved on the team.
good luck this season