how Dads can be friendly with their children?

The best way dads can be friendlier with their kids is by becoming their friends rather than just being a father.
The reason why dads would want to be friends with their kids and not just a father to them is primarily because of the age and generation gap and by becoming friends,  the mental understanding comes to a more or less same level.
So if dads prefer to be best friends to their child, the level of understanding increases to a whole new level. A child vibes the most with his friends, he plays along the most with his or her friend, so if his dad can be their friends,  the regular relation of parent kid formality and the monotonous machinery of correction and teaching. The kids will play and try to learn most from his best friend,  their parents. They'll share all their problems and most importantly the relationship between the father and children strengthens.
The solidarity and bond between the two kins , parent and kid, is maximized when his dad becomes the children's best friend.