This study delves into the concept of Harmonious Bilingual Development (HBD), characterizing families where linguistic situations do not adversely affect their well-being. Following a recently proposed holistic framework on Harmonious Bilingual Experience (HBE), this research aims to discern the relationship between input factors and the bilingual children’s social–emotional and behavioral skills (SEBS). While input undoubtedly plays a major role in bilingual development, more insight is needed on the quantity and quality of input necessary to foster HBD. A total of 36 five-year-old children with Russian and French as their first languages participated in our study. Children’s language exposure and input quality, as well as parental attitudes, beliefs and strategies regarding language transmission, were assessed with parent questionnaires and activity journals. Additionally, parents assessed their children’s SEBS using the Strengths and Difficulties Questionnaire. Hierarchical cluster analysis identified family profiles which support HBD. We found that the quantity of Russian input, as measured by current exposure, does not directly correlate with SEBS. However, less than 30% of weekly input in Russian appears insufficient for achieving HBD. Furthermore, our study suggests that engaging in at least one extra-curricular activity in Russian, e.g., Saturday school, coupled with parental impact belief, aligns with a family profile experiencing more HBD. These results are coherent with previous studies on harmonious bilingualism, and emphasize the interplay of quantitative and qualitative input factors, as well as SEBS, for achieving HBD.