What is the benefit of Test Suite over Child Test Plan

The primary purpose of a Test Suite is to order the execution of a set of Test Cases (where you can use environment variables to use the output of one Test Case as the input to the next Test Case).   A related secondary purpose is to allow you to specify the "roll-up" result for that set of test cases.   The primary purpose of a Test Plan is to plan the testing work of a particular testing team.   So in your case, unless you need to order the execution of the Test Cases in the Regression Test Cases, or need to capture a a roll-up result for the Regression Test Cases, no need to create a Test Suite for them.   If the Regression Test Cases are being handled by a different team from the other Test Cases, then having a separate Test Plan for Regression Test Cases would make sense.   If not, then no need for either a separate Test Suite or Test Plan ... just define a category or custom attribute that indicates whether a Test Case is a Regression Test Case.