The infant being held by his dad, Tony Toponi, in a crowd.
Tony’s baby is a very minor character who appeared in An American Tail: Fievel Goes West and the nameless infant child of Tony Toponi and Bridget. He is first seen when he and his parents were getting away in a roller skate from the cat attack, then the Toponi family were seen in a crowd in the sewer. He and his parents were seen as they moved to a discarded teapot in Green River, which he was last seen in full. He was partially seen with his parents while they and the crowd are on the giant mousetrap.
Notably the Toponi family disappeared in majority during Fievel Goes West as Pat Musick had no involvement, causing her character, Tony Toponi, to be mostly written off, as well as completely in Fievel's American Tails.
After retconning the first sequel, the child fades completely. This issue gained some debate since.