The Bilingual Program of the Community of Madrid (BPCM), Spain, started offering Content and Language Integrated Learning (CLIL), in English, at Primary Education in 2004, at Secondary in 2010, and at Infant Education (3–6-year-old students) in 2017. This approach requires multiskilled practitioners, highly trained in the CLIL methodological principles, to ensure the planning and delivery of effective lessons. However, CLIL teacher training is deficient and needs to be boosted through alternative proposals. This work analyses the potential of a checklist for self-evaluation and observation of CLIL teachers at Infant Education. It is an exploratory research that presents a case study in two Infant Education classrooms in which CLIL is taught by the same teacher. The data collection includes linguistic data collecting techniques such as the checklist for the observer and for the teacher, an observation protocol, transcripts of the interviews, among others. Results from the qualitative analysis of the Infant teacher self-evaluation and the observer showed that a never-ending-teacher-development-awareness to promote teachers to manage their experience adequately was stimulated, and it was likely to open the door to innovation in educational trends (CLIL) in order to offer a solid respond for their professional needs. It also proved to identify the actual CLIL training needs of the Infant teacher and unveiled her thoughts and practice in her bilingual classes. Therefore, it can be concluded that the self-evaluation checklist can be a useful instrument likely to shed some light on the complex phenomenon of in-service CLIL teacher training.