I'm not sure specifically which 'boy and giant snake story' you're referring to. It could be about a boy who has an encounter with a huge snake. Maybe he discovers the snake in a forest or near his home, and there could be elements of adventure, like trying to escape from the snake or befriend it.
The main characters are the boy and the giant snake. The boy could be an ordinary child or someone with special qualities. The giant snake, on the other hand, could be a fearsome creature, but it may also have a complex personality. Their interactions, whether it's a conflict or an alliance, form the core of the story.
No, it's not appropriate. Such a description involves elements that can be disturbing and inappropriate, especially when it comes to fanfic which may be read by a wide range of audiences, including minors.
The name of the movie you asked about is The King of the Snake Throne. The movie was released in 2008 by Disney-Pixar and told the story of a legendary giant snake. In this story, a giant snake king with nine heads led a group of snakes to fight a soul-stirring battle with humans.
I'm not sure specifically which 'boy and giant snake cut out heart story' you mean. There could be various folktales or fictional stories with such elements. Maybe it's a story about a brave boy who has to face a giant and dangerous snake and in a crucial moment, something related to cutting out the snake's heart occurs for some important reason, like to defeat the snake or obtain a magical power.
It depends on where you look. There are many fanfic platforms, and some might have works that reimagine the 'giant snake x female human' concept in a more innocent light. It could involve elements such as the snake having special powers that the female human needs to save her family or village, creating a story full of heroic deeds and a unique relationship.
One possible moral could be courage. The boy had to be very brave to face the giant snake, and his act of cutting out the heart shows that he was willing to take extreme risks for a greater good.