The investigation of User Experience (UX) is a crucial aspect in the development of products and services. Specifically, the study of UX in children and adolescents is essential, as they are in a state of rapid growth and their needs and expectations vary significantly. However, it is challenging to understand the needs and interests of children and adolescents, as they differ from those of adults. Only a limited extent of research has been conducted on the development of UX questionnaires for children and adolescents (eight to 14 years), considering their linguistic and cognitive abilities. This publication focuses on the development of a questionnaire for the target group, considering participatory elements, as well as initial findings on the use and evaluation of interactive apps with younger target groups. The findings of this study provide a foundation for future research in UX research with children. They show that children are not only able to provide valuable insights concerning their needs and preferences for UX evaluation, but also that they can be active participants in the design of UX questionnaires. The developed instrument measures eight UX factors are learning development, overall impression, design and appearance, perspicuity, stimulation, usefulness, quality of content and attractiveness. They can be used in different evaluation scenarios and combined in different ways.