IMPORTANCE: Even brief periods of hardship during early childhood may have lifelong consequences. Prior cross-sectional research limited to respondents with English proficiency and internet access during the COVID-19 crisis documented families with young children that struggled to afford basic needs like food and rent. Few studies have examined experiences of families with young children by race and ethnicity and maternal nativity. OBJECTIVE: To examine the association of COVID-19 relief programs with the mitigation of household food insecurity among families with young children, as well as being behind on rent and disparities in program receipt. DESIGN, SETTING, AND PARTICIPANTS: This cohort study used data from the ongoing repeat cross-sectional Children's HealthWatch study comprising families surveyed at baseline (January 1, 2018, to March 20, 2020) and during the COVID-19 crisis (September 1, 2020, to June 30, 2021). Baseline questionnaires were administered face to face to caregivers of children aged 48 months or younger in English or Spanish in emergency departments or primary care clinics in 5 US cities. The follow-up questionnaires were administered via telephone. EXPOSURES: Supplemental Nutrition Assistance Program (SNAP) participation or receipt of at least 1 Economic Impact Payment (EIP; ie, stimulus check) during the COVID-19 crisis. MAIN OUTCOMES AND MEASURES: The primary outcomes were household food insecurity (assessed via the US Household Food Security Survey Module: Six-Item Short Form) and being behind on rent. Logistic and binomial regression models were used to calculate adjusted odds ratios (aORs) and unadjusted and adjusted prevalence ratios (PRs). RESULTS: Of 1396 caregiver-child dyads (20.3% response rate), race and ethnicity data were available for 1357 caregivers: 514 (37.9%) were Black, non-Latino; 558 (41.1%) were Latino; 230 (16.9%) were White, non-Latino; and 55 (4.1%) were of other non-Latino race or ethnicity. Among 1390 responses with nonmissing data, 417 children (30.0%) had an immigrant mother, and among 1388 responses, 1238 (33.5%) were publicly insured. During the crisis, 467 of 1395 respondents (33.5%) reported household food insecurity, and 567 of 1391 respondents (40.8%) reported being behind on rent. Families with immigrant mothers had lower odds of EIP and SNAP participation than families with US-born mothers (eg, aOR, 0.07 [95% CI, 0.05-0.12] for both EIP and SNAP vs neither), despite being more likely to report household food insecurity (adjusted PR [aPR], 1.48 [95% CI, 1.28-1.71]) and being behind on rent (aPR, 1.14 [95% CI, 1.00-1.30]). Families with Black (unadjusted PR [uPR], 1.40 [95% CI, 1.08-1.82]) or Latino (uPR, 1.54 [95% CI, 1.19-1.98]) caregivers or caregivers of other race and ethnicity (uPR, 1.67 [1.12-2.49]) were also more likely than families with White, non-Latino caregivers to experience household food insecurity or being behind on rent (families with Black caregivers: uPR, 2.02 [95% CI, 1.58-2.58]; families with Latino caregivers: 1.68 [95% CI, 1.30-2.16]; families with caregivers of other race or ethnicity: uPR, 1.94 [95% CI, 1.34-2.80]). Adjustment for covariates and differential participation in relief programs did not entirely account for these disparities. CONCLUSIONS AND RELEVANCE: The results of this cohort study suggest that the COVID-19 crisis exacerbated preexisting inequities in food insecurity and difficulty paying rent according to race and ethnicity and maternal nativity and that equity-focused policy changes are needed to ensure that all children and their families in the US can afford basic needs for optimal health.