The topic of "Hide-and-Seek" is childhood and what the author remembers. He invents this topic by saying what he did in his childhood and how he remembers it. Fulghum tells us how innocent children are. They can play these games and just fun, but adults can't play hide-and-seek, and they can't just have fun.
The author assumes that the audience understands how he feels. He comes straight out and tells the audience that he remembers once how much he had fun and what it was to be a kid and be worry free.
The author's tone is very nostalgic. He expresses his longing to just go out and play games like hide-and-seek. He's an adult now, and adults don't play hide-and-seek. Fulghum says that it's too bad that adults can't play. By the way he expresses his longing it sounds as if he were a child he would be playing hide-and-seek, and he would just be playing around and having fun.
In the introduction Fulghum includes the past and his longing for the past and how things once were. On a deeper note he wants to be free and a child once again because they don't have responsibilities, and they are not always rushing. Being an adult you're doing constantly. You don't time to just play a game and have fun. You don't have time just to yourself to sit down and read a book just for the fun of it. There are so many things as an adult that keep you from just having fun.
If I were to write on this topic I would show more depth. I would show more experiences of being a child, and I would ask how many people miss their childhood and wish they could go back.