Background: COVID-19 restrictions such as the closure of schools and parks, and the cancellation of youth sports and activity classes around the United States may prevent children from achieving recommended levels of physical activity (PA). This study examined the effects of the COVID-19 pandemic on PA and sedentary behavior (SB) in U.S. children. Method: Parents and legal guardians of U.S. children (ages 5–13) were recruited through convenience sampling and completed an online survey between April 25–May 16, 2020. Measures included an assessment of their child’s previous day PA and SB by indicating time spent in 11 common types of PA and 12 common types of SB for children. Parents also reported perceived changes in levels of PA and SB between the pre-COVID-19 (February 2020) and early-COVID-19 (April–May 2020) periods. Additionally, parents reported locations (e.g., home/garage, parks/ trails, gyms/fitness centers) where their children had performed PA and their children’s use of remote/streaming services for PA. Results: From parent reports, children (N = 211) (53% female, 13% Hispanic, Mage = 8.73 [SD = 2.58] years) represented 35 states and the District of Columbia. The most common physical activities during the early-COVID-19 period were free play/unstructured activity (e.g., running around, tag) (90% of children) and going for a walk (55% of children). Children engaged in about 90 min of school-related sitting and over 8 h of leisure-related sitting a day. Parents of older children (ages 9–13) vs. younger children (ages 5–8) perceived greater decreases in PA and greater increases in SB from the pre- to early-COVID-19 periods. Children were more likely to perform PA at home indoors or on neighborhood streets during the early- vs. pre-COVID-19 periods. About a third of children used remote/ streaming services for activity classes and lessons during the early-COVID-19 period. Conclusion: Short-term changes in PA and SB in reaction to COVID-19 may become permanently entrenched, leading to increased risk of obesity, diabetes, and cardiovascular disease in children. Programmatic and policy strategies should be geared towards promoting PA and reducing SB over the next 12 months.