The quality of formal childcare has been cited as an important determinant of young children’s mental health, as it has been demonstrated that children’s self-regulation abilities – resulting from repeated cycles of co-regulation in caregiving – hold extreme importance for their mental health, now and later in life. Given the high uptake of formal childcare in Flanders and the high enrollment rates of the youngest children, childcare teachers thus play an very important role in these co- and self-regulation processes. High job strains within the sector, however, put these services at risk of inducing stress and mental well-being problems among childcare teachers, possibly impacting the co-regulation processes. Fundamental research is scarce in measuring, disentangling and optimizing these relationships. In the current proposal, we will address these gaps using a mixed-method design focusing on 3 research objectives (ROs): (RO1) map the job characteristics that explain job strain in childcare teachers (structural quality), and investigate its association with childcare teachers’ ensuing mental well-being, (RO2) examine the impact of childcare teachers’ strain and ensuing mental well-being on their co-regulation attitudes and practices (process quality), and (RO3) explore effective strategies to structurally optimize both the structural and process quality of formal childcare.