This is what I came up with.
1. Toshiro is a closeted TB who privately acts like a baby to reduce his stress as a captain. One day Rangiku walks into his room and discovers his secret.
2. While in the human world Toshiro gets stuck in a broken gigai and suffers from bedwettings. From there he starts to develop a child like attitude.
3. Rangiku and Toshiro have to pose as Mother and Son in the human world. Rangiku gets too excited about pretending to be a mother that she starts babying Toshiro.
That's all I came up with which do you like best?