when do we the kid transform to gnome in little nightmare

The kid transforms into a Nome in Little Nightmares during the Secrets of the Maw DLC, specifically in The Residence when the Lady catches him after he enters her private area and uses her magic on him. After that, he appears as the Nome that Six later eats in the main game’s ending sequence.

What happens

• The character is the Runaway Kid from the DLC, not the main-game protagonist.

• He is caught by the Lady in The Residence and turned into a Nome.

• That transformed Nome is strongly implied to be the one Six eats later.

Simple answer

So, the transformation happens at the end of the Residence chapter in the DLC , when the Lady discovers him.

TL;DR

The kid becomes a Nome when the Lady catches him in The Residence DLC chapter.