PROJECT SUMMARY — RESEARCH PROJECT 2 (RP2) The American Academy of Pediatrics has endorsed primary care-based screening of mental health risk in young children as a key step toward improving youth mental health, but most early childhood mental health risks go undetected and unaddressed. Even when they receive screening, only 10%-25% of young children identified as “high risk” access mental health care. The availability of transdiagnostic decision tools for risk identification is a major advance; however, increasing uptake of post-screening mental health resources will require a community- partnered approach that centers families' needs, concerns, and experiences with mental health systems. Such approaches must address caregiver preferences for certain types of support (e.g., for briefer, primary care-based resources); address caregivers' beliefs and concerns that constrain help-seeking (e.g., beliefs that mental health interventions will not help their child); and grant agency to caregivers navigating complex support systems. Given the outsized impact of early prevention, there is a need for accessible adjunctive interventions that empower caregivers to increase their likelihood of seeking recommended supports after a positive screening for mental health risk at toddler pediatric well visits. Online single-session interventions (SSIs) have shown promise in increasing caregiver receptivity to mental health intervention and increasing linkage to intervention. For example, a digital (online) “growth mindset” SSI (GM-SSI, developed by Project Lead Schleider) teaching that emotions are malleable has strengthened both child mental health and caregiver support-seeking across multiple trials. Research Project 2 (RP2) of the Mental Health, Earlier (MHE) Center seeks to adapt an online, evidence-based GM-SSI for caregivers of young children (ages 24-30 months) and test its potential to bridge the gap between primary care-based mental health risk screening and families' access to child mental health support. When integrated into the Center's Roll-Out Implementation Optimization design, it will bridge the Signature Project's early identification decision tool and referral to the Family Checkup Online intervention. First, we will use human- centered design methods to ensure the GM-SSI is developmentally and culturally informed for early childhood primary care, via co-design with end-users (racially/ethnically minoritized caregivers) and key informants (pediatricians, administrative staff). Second, via usability testing, we will collect pre-, post-, and follow-up data on mechanisms (beliefs about mental health support; agency; hopelessness) and 3-month clinical outcomes (child mental health support access) of the GM-SSI. Third, we will use these results and conduct a pilot trial involving 30 primary care clinics and 350 caregivers of children screened as showing moderate-or-greater mental health risk. We expect caregivers who receive the adapted SSI (vs. the c...