Mom-of-4, it would help to know: Does your school expect the child to be present at the conference and to be a participant? Or does your school say that parent-teacher conferences are just that -- for the parents and the teacher? As you can see from the replies below, different schools do this in very different ways. Some schools even clearly say that students are not allowed to sit in.
Since you're asking the question the way you do, I assume your school has conferences that are for parents and children are not admitted to the room. If that's the case, it's fine for you to take your child plus an engaging book and have your child sit outside the room, if you can't leave your child home alone. I would always prefer to leave my kid at home doing something constructive but if your child is too young to stay home alone, bring your child but be very clear in advance about what will happen, that your child must stay put, that he or she can't wander the hall or whatever.
Our schools don't include students in the conference. It's like that so that teachers can talk freely and frankly with parents about issues or problems. If the child is sitting right there hearing it all, the teacher likely won't be as open with parents and the parents will hesitate to ask tough questions about their own child. I get that some schools have an approach of "Involve the child" and that may work well at middle school and above, but in elementary, I'd prefer the parent-and-teacher-only conference.