Based on the fact that you have an 8 and 10 yr old accompanying you, I would vote for the Mariner itinerary also. There are limited activities in Belize. Cave tubing is one of the most popular, but most tour companies will not allow children under 10 to participate (some even impose a minimum age of 12). Costa Maya is also somewhat limited, with the Mayan Ruins being the main reason to visit.
Dunn's River Falls can be fun and does not have the same age restrictions as cave tubing. Finally, I always find the private islands to be one of my favorite stops. I have included a link to information on Labadee that is found on our sister site (PortReviews.com). You can also find information on all of these ports in question at that site.
Hope this helps-