Rozumím.
Jenže hlavní potíž je potom, jak zobrazovat každé dítě jen 1x.

Zkusil jsem:

SELECT *, school_class.class_name skola_trida FROM school_class LEFT JOIN child_info ON school_class.id_school = child_info.id_school_class
WHERE (id_school = 100 AND school_class.platne = 1) GROUP BY school_class.id_school ORDER BY school_class.id

Problém: Nezobrazuje prázdné třídy.

Select *, school_class.class_name skola_trida FROM school_class LEFT JOIN child_info ON school_class.id_school = child_info.id_school_class WHERE (id_school =100 AND school_class.platne = 1) ORDER BY school_class.id

Problém: Duplikuje třídy, dělá bordel na prezentační vrstvě - násobí třídy i členy.