It really depends on how the three of you want to sit when you are on the plane. As mentioned before you won't be able to all sit in the same row as seats are three across. If you want to sit with your child (and the empty seat) in one row then I would have her pre-board with you and DH can board at his position and sit wherever. If you want your child to sit with DH then she should board with him at their assigned boarding position.