A parent activity always follows the date range of its subactivities — i.e. it starts on earliest subactivity's start date and finishes on latest subactivity's end date (both start and end dates are bound).

Parent activities are also known as "Summary Activities", in project management speak, because they "summarize" or aggregate the info, within their child activity group. This is not limited to star/end dates / duration but also estimated hours, actual hours, budget info, etc.

Note: this is different from dependencies, whereby task dates are set by predecessors or master tasks - for summary tasks there is no need to set this start/end date relationship - it's built-in and as such it's Never a good idea to set dependencies on Parent Activities as this can cause a circular reference!