As a midwife, it is ideal for me to bring an assistant. This frees me up to take care of you! I will either bring one or two depending on who is available and if I am training. Midwifery care is a hands on education in many ways therefore I believe it is important to pass along the education to students who are wanting to learn more.
The short answer is that your family can be as involved as feels right to you! I enjoy having children present for prenatal visits and births. I think it's important for the whole family to have an opportunity to be a part of welcoming the newborn. I strongly recommend that you have someone just for your child or children who can be completely present and responsive to their needs and questions. Of course, it should be someone you are all comfortable with. There are wonderful books and materials for preparing children for the sights and sounds of labor and birth, and there are many videos too. Children are not yet socialized to fear birth, and whether they sleep through the night and wake to find a new sibling, or actively take part in welcoming the new baby, homebirth takes place within the natural rhythm of the family.