Fruit drinks are widely consumed by young children, and many parents mistakenly believe that these drinks are healthy, potentially due to front-of-package claims and imagery. Research is needed on the influence of this marketing and how labeling regulations could change behavior. To assess the effects of a front-of-package 100% vitamin C claim, fruit imagery, percentage juice and teaspoons of added sugar disclosures, and high-added sugar warnings on parents' choices, knowledge, and perceptions of beverages. This randomized clinical trial was conducted May to July 2021 as a single-exposure (no follow-up) online survey of primary caregivers of children ages 0 to 5 years throughout the US. Participants were shown no-, low-, and high-added sugar beverages and asked to choose 1 for their child. Participants were randomized to see high-added sugar beverages with 1 of 7 front-of-package conditions: (1) claim and imagery (control); (2) no claim; (3) no imagery; (4) no claim or imagery; (5) claim, imagery, and percentage juice disclosure; (6) claim, imagery, and warning; or (7) claim, imagery, warning, and teaspoons of added sugar disclosure. Primary outcomes were type of beverage chosen (eg, high-added sugar beverage) and resulting calories and added sugar (in grams). Secondary outcomes were fruit drink knowledge (added sugar and percent juice) and perceptions. There were 5005 participants included in the final analysis (mean [SD] age, 31.5 [8.3] years; 3587 female participants [71.7%]), including 714 participants in group 1, 717 participants in group 2, 710 participants in group 3, 717 participants in group 4, 708 participants in group 5, 729 participants in group 6, and 710 participants in group 7. Compared with participants in the control group, who had a mean (standard error [SE]) of 9.4 (0.5) g of added sugar and 81.9 (1.6) kcal in chosen beverages, only participants who saw warnings with teaspoons of added sugar disclosures had significantly reduced added sugar (-1.3 g; 95% CI, -2.6 to -0.1 g [-14.2%; 95% CI, -26.7% to -1.8%]; P = .04) and calories (-5.3 kcal; 95% CI, -9.8 to -0.9 kcal [-6.5%; 95% CI, -11.8% to -1.3%]; P = .02) in selected beverages. In warning conditions (ie, 6 and 7) compared with the control group (mean [SE] 41.0% [1.8%]), the proportion of participants choosing high-added sugar beverages was significantly reduced, by 5.5 percentage points (95% CI, 0.5 to 10.5 percentage points [13.4%; 95% CI, 1.2% to 25.6%]; P = .03) and 6.4 percentage points (95% CI, 1.4 to 11.4 percentage points [15.6%; 95% CI, 3.3% to 27.8%]; P = .01), respectively. The no claim or imagery condition (4) significantly reduced the proportion of parents choosing high-added sugar beverages (-7.6 percentage points; 95% CI, -12.6 to -2.6 percentage points [-18.4%; 95% CI, -30.6% to -6.3%]; P = .003). Percentage juice disclosures did not affect beverage choice. These findings suggest that added sugar warnings and prohibitions of front-of-package claims and imagery may reduce parents' purchases of high-added sugar beverages for their young children but that percentage juice disclosures may not change behavior. ClinicalTrials.gov Identifier: NCT04811690.