Baby Tanjiro and Nezuko with their parents! This is the youngest we ever see them. (More precisely, he's probably around 1 year old here.)
(Volume 1, Chapter 1)
One of Gotouge’s unsung writing strengths is choice of using endearments at critical times. Most know of Zenitsu’s use of Aniki when he goes to slice Kaigaku’s head, there’s also Kanao never using Nee-san until Douma ask who she is and she finally say Kocho’s little sister
This is the only scene where Tanjiro calls his mother 母ちゃん (mommy) instead of 母さん (mom).
Knowing Gotouge, it was likely a deliberate choice to emphasize that he's still just a child that needs its mother, regardless of how mature he tries to act
(Volume 1, Chapter 1)