High levels of stress are frequently observed in children during hospitalization. Among available stress management tools, virtual reality (VR) represents a promising option due to its immersive, engaging qualities. While many VR platforms exist, most rely on passive distraction; few incorporate guided breathing exercises. Objective: This pilot study aimed to develop and assess the feasibility and acceptability of two immersive virtual environments (VEs) designed to promote relaxation in children through nature-inspired landscapes and guided diaphragmatic breathing. Before conducting randomized trials in clinical settings, a preliminary evaluation of the key design elements, essential for developing effective VR experiences in health care, is necessary. Methods: A total of 105 healthy children aged 6 to 15 years experienced both VEs. Feasibility and acceptability were assessed through perceived usefulness, satisfaction, cybersickness symptoms, and sense of presence. Anxiety levels before and after immersion were also measured. Results: Findings showed high satisfaction and a strong sense of presence for both environments, with minimal cybersickness reported. Preliminary data suggest a potential reduction in postimmersion anxiety. Age and gender effects on these outcomes are also discussed. Conclusions: This study provides initial evidence supporting the acceptability and feasibility of these nature-based VEs incorporating breathing exercises. It represents a fundamental step within a broader iterative process of design, evaluation, and optimization. The findings offer valuable insights to guide the refinement of future iterations of these environments, while also raising considerations for their potential clinical implementation.