<p>Children under the age of six are disproportionately exposed to traumatic experiences and seem especially vulnerable. Trauma often affects both children and caregivers and their relationships. Trauma-focused treatment and its long-term effects for young children are of prime interest, but research is limited and lacks follow-up data. The current study explored the long-term effects of Child-Parent Psychotherapy (CPP) treatment and potential predictors of outcome. The sample included 37 traumatized young children, aged 2–6 years, who had received the dyadic treatment together with their caregiver in a multi-site clinical setting. The majority had been exposed to several potential traumatic events, including interpersonal trauma. The study was a naturalistic one-group, pre-post design study with a 6-month follow-up. Outcome measures comprised child and caregiver post-traumatic stress symptoms and signs of caregiving disorganization reported by caregivers. Piecewise Linear Mixed Models were applied to explore long-term treatment effects. Within-group effect sizes were calculated using model-estimated differences in mean values. Possible predictors of outcome were analyzed by adding them as covariates in the model and interacting them with time. The outcomes remained consistent six months after treatment. Positive effects were reduced child and caregiver post-traumatic stress symptoms (<i>d</i> = 0.62; <i>d</i> = 0.57, respectively) and signs of caregiving disorganization (<i>d =</i> 0.64). A higher degree of child trauma symptoms predicted less reduction in caregiver traumatic stress. The results indicate that children, caregivers, and their relationship benefit from CPP and that results are sustainable. The naturalistic design strengthens the applicability of CPP.</p>