I don't know for sure, but Tompkin is probably the main character in the story. It could be a little boy, a small animal like a mouse or a rabbit.
Tompkin in the 'Book Tompkin Coming Out at Night' kids story is most likely the protagonist. It's hard to say exactly what or who he is without reading the story. However, given that it's a kids story, he could be a child - like character, perhaps an anthropomorphic animal. He might be a character who is brave enough to venture out at night, when most are asleep. His name gives the impression that he has a unique identity within the story, and he is likely the one through whom the story unfolds, whether it's through his adventures, his discoveries, or his interactions with other characters in the night - time setting.