Sheldon soothes baby Constance after Mandy's efforts to calm her own daughter causes her to give up on it. Surprisingly, the usually socially-inept kid is able to pacify his niece by singing "Soft Kitty" to her. While this moment is arguably one of the sweetest in season 6, it also reinforces an existing Young Sheldon/TBBT plot hole about when it can be sung as Sheldon claimed that it is only during one is sick.
Penny asked for Sheldon to sing the infamous song as she was nursing her dislocated shoulder. Sheldon refused, however, claiming that "Soft Kitty" is only sung when one is sick. Clearly, baby Constance isn't sick in Young Sheldon; she has just been crying a lot and Mandy just hasn't figured out a way to pacify her. Sheldon's refusal to accommodate Penny's request highlight just how callous and indifferent he was to her during the early years of their friendship. Over time, however, he started trusting and caring for her.