Neglected Child Sees an Imaginary Zombie/Clown

There is a boy, probably five or six. His parents always leave him for work. I think he develops an imaginary monster who is a clown or a zombie (I can’t tell which one of the two) inside his head because of being neglected.

He tells his parents but they don’t believe him. I think for the ending, it is his birthday and there are guests, and he keeps telling his parents about the monster, but they still don’t believe him. He is the only one who sees the monster.

The monster always smiles and waves at the boy. And the boy has black hair.

These are all the details I can remember. Thank you for reading this. Hoping you can help me if you have time. 🩷

Is it Z?