Parent table
id  married_couple  married_at  expected_kids
--------------------------------------
1   Bob and Mary    2010-01-01  4

Child table
id  child_name  parent_id  date_of_birth
--------------------------------------
1   Eddie       1          2012-01-01
2   Frankie     1          2013-01-01
3   Robbie      1          2014-01-01
4   Duncan      1          2015-01-01

Expected SQL result
parent_id  half_goal_reached(years)
--------------------------------------
1          3