Welcome to Cruise-Chat, Bonnibren. You and your family are in for an amazing experience, not only the ship itself, but the ports you are visiting are fantastic!
We did the dolphin swim in Chankanaab Park. I don't care where you do it, just do the swim make sure it's either the swim or the royal swim and not the encounter. The encounter does not let you interact with the dolphins, the swim does. We enjoyed Chankanaab Park and after the swim, we did a little snorkeling from the shore. We enjoyed a couple of drinks at the restaurant/cantina, bought the video of our swim and returned to the ship. I'm sure the other place is nice, too.
I've read very good reviews of the Chacchoben ruins, but have not read anything about the other ones. Probably because Chacchoben is closer. We did the Tulum ruins in Cozumel on one trip, but it is an all day process and most of it is spent getting there. The setting is beautiful and the ruins are complete, but the dolphins would be a better day, in my opinion.
In Roatan we did a snorkel trip through the ship that was fabulous. Crystal clear water, good sea life and good coral formations. There have been good reviews on Foster's beach and Tabayana beach. If you'll look through our Caribbean Port Chat section, a little further down on the main page and use your "find" feature in the upper left hand corner; type in Roatan, and it will pull up all the posts made about it. Maybe something there will interest you.
I think bringing you clothes for formal night not pressed is a great idea. Their service is not that expensive, and yes, do it the first night. More than likely your formal night will be your second night.
Hope this helps some! Best wishes!