Individuals with Down syndrome (DS) exhibit delays in motor, cognitive, language, and social development. Neural impairments lead to significant biomechanical challenges, particularly in stabilizing lower extremity joints. As a result, children with DS often develop compensatory strategies, such as co-contraction of antagonistic muscles, contributing to inefficient gait mechanics. Supramalleolar orthoses (SMOs) are commonly prescribed to provide support to the ankle above the malleoli, enhance joint stability, and improve gait. However, evidence on their effectiveness, particularly in relation to obstacle crossing mechanics and strategy selection, remains limited. Consequently, clinicians often rely on anecdotal guidance regarding the timing and necessity of SMO use. This study examined overground walking and obstacle crossing in 23 young children with DS (ages 2–6), comparing 10 daily SMO users to 13 non-users. Children were evaluated while walking barefoot and with footwear, crossing obstacles set at 10% and 30% of leg length. We assessed various crossing strategies, including successful and unsuccessful steps over the obstacle, bear crawling, walking around, or refusing to cross. Results showed that SMO users demonstrated improved overground walking mechanics when wearing their orthoses, including increased cadence, walking speed, step length, and hip and knee angular velocities. During obstacle crossing, SMO use was associated with higher rates of successful crossings and fewer secondary strategies (e.g., walking around or refusing). Additionally, the SMO group showed improved step length and crossing velocity with SMOs on, though they relied more on frontal plane hip motion and proximal joint strategies to clear the obstacle. In conclusion, SMOs can enhance walking and obstacle crossing performance in young children with DS. However, prescription decisions should be individualized, taking into account specific characteristics such as joint laxity and hypotonia. These findings provide clinicians with data-driven insights into the functional benefits of SMOs and may inform more targeted orthotic interventions.