Despite custodial grandchildren’s (CG) traumatic histories and risk for psychological difficulties, knowledge is scant regarding the frequencies, types, and consequences of adverse childhood experiences (ACEs) they have encountered. We examined self-reported ACEs via online surveys with 342 CG (ages 12 to 18) who were recruited to participate in an RCT of a social intelligence training program. ACEs were assessed by 14 widely used items, and risk for internalizing (ID) and externalizing (ED) difficulties were measured using 80th percentile cut-offs on the Strengths and Difficulties Questionnaire. Classification and regression tree analyses included all 14 ACEs (along with CG gender and age) as predictors of ID and ED risk separately. Given possible comorbidity, analyses were run with and without the other risk type as a predictor. Less than 9% of CG self-reported no ACEs, 48.6% reported two to five ACEs, and 30.5% reported ≥6. Irrespective of ED risk, bullying from peers strongly predicted ID risk. ED risk was peak among CG who also had risk for ID. Without ID risk as a predictor, ED risk was highest among CG who were emotionally abused, not lived with a substance abuser, and encountered neighborhood violence. The frequency and types of ACEs observed were alarmingly higher than those among the general population, suggesting that many CG have histories of trauma and household dysfunction. That a small number of ACEs among the 14 studied here were significant predictors of ID and ED risk challenges the widespread belief of a cumulative dose ACE effect.