I created a basic version of your setup and, indeed, the parent and child attack simultaneously; so, there’s no issue with the parent’s IsAttacking boolean value propagating to the child. I didn’t need the OnSeePawn node in my child bp, so that can be removed.
But, to the actual issue, the only other thing I can think of is a collision issue. Perhaps one of either the collision sphere or scene component on your parent (or another on your player) is blocking the view of your child. Check the attached video. A quick check would be to move, scale, or disable the block of pertinent channels.