The concept of child well-being has been studied in a variety of disciplines. However, the well-being of children is often based on the experience of parents, professional assessments or health surveys. The voice of children and the subjective experience of their well-being is often missing. We need more research and indicators of children's subjective well-being. Services for children and families are being reformed in the Finnish Government's key project, Programme to Address Child and Family Services (LAPE). Laurea University of Applied Sciences is studying the experimental well-being data of small children in the project and is developing the subjective well-being indicator for children. In this article, the authors consider why children's subjective well-being information is needed and what should be known about the subjective well-being of children. The aim of the research is to discuss the essential considerations when assessing the subjective wellbeing of children.