Parents play a substantial role in their children's emotion regulation (ER) abilities. Children with autism spectrum disorder (ASD) often have difficulties regulating their emotions, which can manifest as externalizing behavioral issues. Parents of children with ASD facilitate their children's ER development in response to unique challenges and stressors, often developing resiliency but other times contributing to their children's dysregulation and behavioral challenges. It is unclear how much momentary stress attenuates parents' emotional facilitation of child ER and its effect on child behavioral functioning. Using an ecological momentary assessment approach, the current study explores this process by considering how parents of children with ASD facilitate child ER through (1) parent ER ability, (2) emotion socialization style, and (3) expressed emotion. Multilevel models explored the relative influences of parent ER facilitation on the association between momentary parent stress and behavioral intensity. Results suggest that parent ER abilities and specific emotion socialization styles interact with momentary parent stress and child behavior, whereas emotional climate of the home impacts child behavior more directly. The role of parent ER facilitation in the context of parent stress and children with ASD and behavioral problems is complex and multifaceted. Implications for further supporting children's emotional development via parent involvement are discussed.