This study examined how early parental involvement in interventions and the quality of the parent–teacher relationship are related to and predict the development of children with ASD, simultaneously assessing their joint contribution. The research involved 69 parents and 69 kindergarten teachers of children with ASD in Israel, utilizing questionnaires to measure parental involvement dimensions, parent–teacher trust and communication, and child development in areas such as language, sociability, sensory/cognitive awareness, and physical health/behavior. The findings revealed a complex picture. While general parental involvement did not directly predict overall child development, specific parental involvement subscales, such as interest/presence and communication patterns, showed significant positive correlations with certain developmental areas, like physical health/behavior and language. From the parents’ perspective, the quality of their relationship with teachers positively correlated with and predicted their child’s overall development, including language/communication, sociability, and physical health/behavior. Teachers’ responses did not yield these significant correlations or predictive power. The study confirmed its main hypothesis: early parental involvement and the quality of the parent–teacher relationship, when considered together, significantly predict the development of children with ASD, accounting for approximately 9% of the variance. These results underscore the importance of a collaborative and coordinated approach between parents and educators to foster optimal development in children with ASD.