Yes, they are two separate things.
For the Free at Sea, read the Terms and Conditions which detail the gratuity charges.
https://www.ncl.com/about/terms-and-conditions/promotions
Daily Service charge info from the NCL FAQ's.
https://www.ncl.com/faq#!#what-is-onboard-service-charge
(under "Money Matters" in the FAQ's)
Just an FYI...you'll find lots of very good tips and tricks on the Family Cruising forum. Here's the link for you:
https://boards.cruisecritic.com/forum/28-family-cruises/