Among the clients in Middle Atlantic, how many are them are female and no more than 18 years old?
in Middle Atlantic refers to division = 'Middle Atlantic'; female refers to sex = 'Female'; no more than 18 refers to age < 18
SELECT COUNT(T1.sex) FROM client AS T1 INNER JOIN district AS T2 ON T1.district_id = T2.district_id WHERE T2.division = 'Middle Atlantic' AND T1.sex = 'Female' AND T1.age < 18