Young children with intellectual and developmental disabilities are at increased risk of developing persistent mental health and behavior problems. While the link between parenting behavior and the development of problem behavior is well understood in this population, there is a need for examination of key parent factors that affect parenting behavior and child problem behavior over time in families of children with developmental delay (DD). Private events such as parents’ feelings about their children and levels of dispositional optimism may impact parenting behavior through a variety of mechanisms, including experiential avoidance and relational schemas. As such, this study proposed to examine relations between parent feelings, optimism, parenting behavior, and child problem behavior for young children with developmental delay in a longitudinal context. Parents’ positive and negative feelings about their young children with developmental delay, dispositional optimism, and child problem behavior were assessed at three timepoints in 132 parent-child dyads. In addition, measures of observed effective parenting behavior during parent-child play interactions were collected at each timepoint. Negative feelings about the child significantly predicted child problem behavior across timepoints, with higher negative feelings predicting higher problem behavior. Positive feelings and optimism did not significantly predict problem behavior in the model including negative feelings, suggesting that correlations between these constructs and reduced problem behavior are primarily explained by reduced negative feelings. Increased negative feelings also significantly predicted a lower rate of praise across timepoints, indicating that parents with high negative feelings about their child with DD engaged in fewer praise statements during parent-child play interactions. These findings suggest that a strong and stable relationship between negative feelings and child problem behavior is present at a very early age for young children with developmental delay and that negative feelings may impact parents’ use of effective and positive parenting strategies. Future research should examine interventions designed to address both parent private events and child problem behavior as well as how these constructs develop both earlier and later in life.