Background: Promoting health literacy in early life is regarded as an important means of sustaining health literacy and health over the life course. However, little evidence is available on children's health literacy, partly due to a scarcity of suitable measurement tools. Although there are 18 tools to measure specific items of health literacy for people younger than age 13 years, there is a lack of comparable, valid, and ageappropriate measures of generic health literacy. Objective: This study aimed to develop and qualitatively test an age-adapted version of the European Health Literacy Survey Questionnaire (HLS-EU-Q) for Germanspeaking children age 9 and 10 years. Although validated for adults and adolescents, the HLS-EU-Q has never been age-adapted or used with children. Methods: The content and language of HLS-EU-Q items were adapted for this age range. The literature was consulted to inform this process, and adaptations were developed and selected based on consensus among authors. From an item pool of 102 adapted items, 37 were given to 30 fourth-grade students in a cognitive pretest, which is a standard procedure in questionnaire development aiming to explore how items are interpreted. Participants (18 girls, 12 boys) were mostly age 9 or 10 years (range, 9-11 years). Key Results: Problems with misinterpretation were identified for some items and participants (e.g., items designed to assess participants' perceived difficulty in accessing and appraising health information were partly answered on the basis of knowledge and experience). A final selection of 26 wellperforming items corresponded to the underlying HLS-EU-Q framework. Conclusions: This is the first ageadapted version of the HLS-EU-Q. A preliminary 26-item questionnaire was successfully developed that performed well in a cognitive pretest. However, further research needs to verify its validity and reliability. The present findings help to advance the measurement of generic self-reported health literacy in children and highlight the need for cognitive pretesting as an essential part of questionnaire development. [HLRP: Health Literacy Research and Practice. 2020;4(2):e119-e128.] Plain Language Summary: The European Health Literacy Survey Questionnaire is used for testing adults' health literacy. It was adapted for German-speaking children age 9 and 10 years. Based on a review of the original items and the literature, 26 questionnaire items were developed and tested in interviews with 30 children. Although problems with understanding could be identified, the questionnaire was mostly well understood.