Yes, I missed the relationship too, because I didn’t recognise とーちゃん which よつば uses on page 6.
とーちゃん is a very casual form of とうさん, which is a casual form of お父さん / おとうさん.
I call my father-in-law お父さん, my wife calls him とうさん, but no one in the world calls him とーちゃん!