Despite evidence of improved insurance coverage under the Affordable Care Act and Medicaid expansion among adults with cancer, little is known regarding the association of these policies with coverage among children with cancer. To assess the association of early Medicaid expansion with rates of Medicaid coverage, private coverage, and no uninsurance among children with cancer. This cross-sectional study used data from the Surveillance, Epidemiology, and End Results (SEER) database from January 1, 2007, to December 31, 2015, to identify children diagnosed with cancer at ages 0 to 14 years in the United States. Data were analyzed from July 27, 2017, to October 7, 2019. Changes in insurance status at diagnosis after early Medicaid expansion in California, Connecticut, Washington, and New Jersey (EXP states) were compared with changes in nonexpansion (NEXP) states (Arkansas, Georgia, Hawaii, Iowa, Kentucky, Louisiana, Michigan, New Mexico, and Utah). Difference-in-differences (DID) analyses were used to compare absolute changes in insurance status (uninsured, Medicaid, private/other) at diagnosis before (2007 to 2009) and after (2011 to 2015) expansion in EXP relative to NEXP states. A total of 21 069 children (11 265 [53.5%] male; mean [SD] age, 6.18 [4.57] years) were included. A 5.25% increase (95% CI, 2.61%-7.89%; P < .001) in Medicaid coverage in children with cancer was observed in EXP vs NEXP states, with larger increases among children of counties with middle to high (adjusted DID estimates, 10.18%; 95% CI, 4.22%-16.14%; P = .005) and high (adjusted DID estimates, 6.13%; 95% CI, 1.10%-11.15%; P = .05) poverty levels (P = .04 for interaction). Expansion-associated reductions of children reported as uninsured (-0.73%; 95% CI, -1.49% to 0.03%; P = .06) and with private or other insurance (-4.52%; 95% CI, -7.16% to -1.88%; P < .001) were observed. For the latter, the decrease was greater for children from counties with middle to high poverty (-9.00%; 95% CI, -14.98% to -3.02%) and high poverty (-6.38%; 95% CI, -11.36% to -1.40%) (P = .04 for interaction). In this study, state Medicaid expansions were associated with increased Medicaid coverage in children with cancer overall and in some subgroups primarily owing to switching from private coverage, particularly in counties with higher levels of poverty but also through reductions in the uninsured.