In children with a hearing loss who receive cochlear implants (CIs) under the age of 2, regular assessments are conducted to monitor auditory and linguistic progress. However, the collection of authentic, representative, and reliable expressive language data on young children with CIs remains a challenge. The purpose of the study was to determine whether data from parental report, language diary, and spontaneous language sample are equally representative of the development of expressive vocabulary over the first 12 months of CI use. Nine French-speaking children and their families participated in the study. We collected data at 3, 6, 9, and 12 months post-implantation, and we measured parental satisfaction regarding the use of a language diary. All three methods showed a progression in the total number of different words expressed over time and captured grammatical diversity. The findings suggest that when vocabulary size is still small, the diary might provide a more comprehensive picture of development than a vocabulary checklist.