Extended Reality (XR) has already been used to support interventions for autistic children, but mainly focuses on training the socioemotional abilities of children requiring low support. To also consider children requiring substantial support, this paper examines how to design XR applications in order to expand clinic-based sensory strategies that are often used by practitioners to put them in a secure state, and how to maximize the acceptability of such applications among practitioners. To that respect, a "Mixed Reality platform for Engagement and Relaxation of Autistic children" was designed and developed, which allows to add audio, visual and haptic individualized or common stimuli onto reality. A first Augmented Reality freeplay use case called Magic Bubbles was created based on interviews with stakeholders and on a collaboration with three practitioners. A preliminary study with eleven practitioners confirmed its well-being potential and acceptability. XR design guidelines are finally derived.