Abstract: A liner (200) for the waste receiving container (104) of a toddler potty trainer seat (100) comprises a liquid impervious outer shell (201) having exterior dimensions closely matched to the interior dimensions of the waste receiving container (104). An elastic band (202) extends around the upper end of the liquid impervious outer shell (201) to secure the liner (200) in engagement with the container (104). A liquid retaining layer (203) extends across substantially the entirety of the interior surface of the liquid impervious outer shell (201). An inner layer (204) extends across the entirety of the liquid retaining layer (203) for passing liquid received in the liner (204) into the liquid retaining layer (203) and for changing color in response to use.