Background: Aquatic therapy (AT), immersion in hot water, and supported standing are frequently used to manage spasticity, contractures, and joint retractions in children with cerebral palsy (CP). Recently, the use of exoskeletons has been offering a new treatment option for severe CP. This study aimed to compare the post-immediate effects of four treatments on spasticity, range of motion, and the heart rate of children with severe CP. Methods: Three children with spastic CP (levels IV and V GMFCS) received a single 30-min session in consecutive weeks of robot-assisted gait training (RAGT), AT, supported standing, and immersion in hot water. Post-immediate assessments included knee flexor spasticity (modified Ashworth scale, MAS, and modified Tardieu scale, MTS); knee range of motion (ROM, in degrees (°)); and heart rate (HR). Results: AT and supported standing induced greater reductions in spasticity based on MAS scores. RAGT demonstrated superior spasticity reduction using MTS and yielded the greatest improvement in popliteal angle (mean increase: 27°). AT and RAGT induced a 14 beats-per-minute change in HR, indicating moderate cardiovascular engagement. Conclusions: RAGT appears particularly effective in improving spasticity and ROM in children with severe CP. Nonetheless, conventional treatments still offer an effective option when addressing spasticity.