This study on the effect of a VR headset on young people (under 10 years old) is based on a 50 children. The result shows that one hour use of VR induce not effect (directly or after a period of time).
Young children tolerate fully immersive 3D virtual reality game play without noteworthy effects on visuomotor functions. VR play did not induce significant post-VR postural instability or maladaption of the vestibulo-ocular reflex. The prevalence of discomfort and aftereffects may be less than that reported for adults.
Source: Effects of Immersive Virtual Reality Headset Viewing on Young Children: Visuomotor Function, Postural Stability, and Motion Sickness. – PubMed – NCBI