I have worked on similar project.
Having different styling for subsite is not a bad thing, and at the same time it is not advisable and recommended to have it with drastic visual design changed, Here are some of the reasons why:
- There is already a parent website that is familiar with your target audience, if they are used to using it they wouldn't appreciate a new look unless it doesn't take away their freedom of operation and emotions.
- The effect of point 1 is, you loose trust on your parent website usage by certain % initially that may affect your purpose of website.
- There are Design and Developer challenges if you change drastically and time taken for development will be appreciably more.
- This increases your target delivery time,delivery dates and also you need to create a style guide, else you might end up in trouble in syncing the vision, design and overall harmony of website with your developer team.
So, my recommendation would be to sync with the parent website. Drastic change can be good experience in terms of designing but ideally should not be the cost of problem solving.
So, you put on points to each and decide.