Many parents I hear from are so busy trying to be a friend to their kids that they actually forget about being a parent. Unfortunately, with many people feeling guilty about how much time they spend away from their kids, they feel a need to bond with their kids as friends.
The first responsibility of a parent is to parent the child. Parenting is a job, and this sometimes makes us the bad guy, we are the ones saying no and bearing the responsibility of guiding our children. Parents would not be necessary if kids could make these decisions for themselves. The lack of maturity in children is the reason they need parents, a mature perspective, to teach them to make the right decisions. Of course, all children think they can do this without parents' help, even we thought this at one time.
Children do not see situations in the same way adults do. They are not yet wired this way. They thrive in their own little world, but we can see outside this perspective. Letting your child make age appropriate decisions will help them grow into the role of making decisions on their own. You may have to guide them a little along the way.
So the answer to the question whether you can be your child's friend is no, you cannot. You should keep lines of communication open and well used, be honest with them, but you have control in the relationship. This means you will have to exercise that control from time to time, and this is what makes a health relationship, not a friendship.
Creating and maintaining boundaries with your child is sometimes hard, but necessary. To be a parent, you have to balance discipline with love, and be willing to admit that everyone makes mistakes. The fact that you are concerned enough to read information on being a parent shows that you are on the right path. Keep on trucking.