Indian travelling to hongkong and want to visit china from hongkong

Hi....i stay in hongkong....my parents from india planning to visit me next month....they want to travel to china from hongkong.... they are taking chinese tourist visa from india...for which they are asked to show air tickets to china from india.... but they have air tickets for hongkong only....they will travel to china from hongkong by train.... Is it ok to submit air tickets to hongkong for application? Thanks