The ongoing war in Ukraine is expected to negatively impact the mental health of the country's population. This study aims to provide a preliminary estimate of the degree of change in the mental health problems of Ukrainian children following Russia's invasion in February 2022, and to identify the sociodemographic and war-related risk factors associated with these changes. A nationwide, opportunistic sample of 1238 parents reported on a single randomly chosen child within their household as part of The Mental Health of Parents and Children in Ukraine Study. Data were collected between July 15th and September 5th, 2022. Participants completed modified versions of the Pediatric Symptom Checklist (PSC-17) which was adapted to capture change in the frequency of symptoms since the beginning of the war. Parents reported increases across all 17 indicators of internalizing, externalizing, and attention problems of the PSC-17. Increased problems were most pronounced within the internalizing domain, with 35% of parents reporting that their child worried more since the beginning of the war. A number of individual, parental, and war-related factors were associated with increases across the three domains. Exposure to war trauma, pre-existing mental health problems, and child age were among the strongest predictors of change. This survey provides preliminary evidence that the Russian war on Ukraine has led to an increase in common mental health problems among children in the general population. Further research is required to determine the extent and sequela of this increase, and to develop intervention strategies for those most in need.