Childhood sweetheart is a reciprocating phrase for a relationship (but not a partnership) between young persons.
This experience forms the basis of subsequent future relationships later in childhood and/or adulthood.
Usually, an individual will have no more than one childhood sweetheart as this term is indicative of a milestone in the growth, development and maturity of a young person.
In ideal circumstances, the term applies mutually to both parties and corresponds both ways, hence the plural being childhood sweethearts.
Occasionally in later years, these friendships are "rekindled" following separations or passing of their adult partners which lead to a later life marriage, union etc.