In order to fulfill the basic functions of our service, the user hereby agrees to allow Xiaomi to collect, process and use personal information which shall include but not be limited to written threads, pictures, comments, replies in the Mi Community, and relevant data types listed in Xiaomi's Private Policy. By selecting "Agree", you agree to Xiaomi's Private Policy and Content Policy .
The free Mi Cloud storage is tagged to the Mi ID. Therefore, 1 Mi ID will be limited to the amount of storage regardless of the number of devices. As what JJ suggested, it is more feasible to get your parents to register a Mi ID for themselves.