We examined the influencing factors of the undermet care needs of the Chinese disabled oldest old people when their children are both caregivers and are themselves older people. Data were obtained from a cross-sectional survey: the Chinese Longitudinal Healthy Longevity Survey (CLHLS) in 2018. The study participants included 1617 disabled oldest old people whose primary caregiver were their children or children-in-law and were aged 60 years and over. The results showed that the prevalence of undermet needs remained high, with 49.6% disabled oldest old people reporting undermet care needs. Binary logistic regression analysis revealed that living in a rural area (OR = 1.309, 95% CI = 1.133–1.513) and a higher frailty index (OR = 1.103, 95% CI = 1.075–1.131) were significantly positively associated with higher odds for undermet care needs, while a higher annual household income (OR = 0.856, 95% CI = 0.795–0.923), more financial support from children (OR = 0.969, 95% CI = 0.941–0.997), higher care expenditures (OR = 1.044, 95% CI = 1.002–1.088), better caregiver’s performance (OR = 0.282, 95% CI = 0.196–0.407) and sufficient income to pay for daily expenses (OR = 0.710, 95% CI = 0.519–0.973) were significantly inversely associated with higher odds for undermet care needs. This evidence suggests the importance of policies to establish a community-based socialized long-term care system and supporting family caregivers of the disabled oldest old people.