wet nurse
姆 centers on a woman who nurses and cares for a child: a wet nurse.
姆 combines 女 (woman) with 母 (mother), suggesting a woman who acts as a mother figure, specifically a wet nurse.
A woman 女 beside a mother 母: picture a wet nurse, a woman who provides motherly care.
For ボ, imagine a wet nurse named Bo: Bo -> ボ, and she cares for the child like a mother.