Does your child receive benefits from the education and participation package (http://www.bmas.de/DE/Themen/Arbeitsmarkt/Grundsicherung/Leistungen-zur-Sicherung-des-Lebensunterhalts/Bildungspaket/leistungen-bildungspaket.html) you get a subsidy for lunch at school or lunch is free of charge.
Please contact the school of Your child for further details.