There is nothing wrong with wanting to be your child's friend. Do what it takes to earn their friendship—be supportive and kind and honest and trustworthy and caring and generous and loyal and fun and interesting and interested in them and all the other things that good friends are to each other. Be the best 40 year old friend you can be (or whatever age you are).
People use "I'm the parent, not a friend," as an excuse to be mean, selfish, and lazy. Instead, be the adult in the friendship. Be mature. You've BEEN a five-year-old and your child has not been a forty-year-old, so you have an advantage in terms of long-term and wider perspective. Use that advantage to be an even better friend. You know how to be kinder and less self-centered and you know how beneficial it is to put forth the effort.
—Pam Sorooshian
photo by Sandra Dodd, of six-year-old Adam and his mother and friend, Julie