Qualitative research is based on longitudinal studies relying on the essential values of the humanist psychology and more precisely the person-centered approach (PCA). The fieldwork of this thesis is carried out with mentally disabled children. The concept of person takes a crucial place in our working plan which focuses on the subjective experience of children and their mental health. In order to operationalize the PCA, we chose to work with two tools, i.e. the "Three Trees" test (TTT) and the "Notebook of the Tree". Afterwards, we analyzed the positive impact of the psychological mechanisms implied in these tests. The child's resources are indeed mobilized thanks to them, and this mobilization consequently reduces personality disorders (and can even suppress them) and increases the activation of the cognitive processes. The psychological accompaniment that we used in our work promotes a theory which respects and acknowledges the subject. It also makes the person respectable and recognizes his or her ability to change.
