The resort will tell you what you need at the restaurants. They really vary, and many have a variety of restaurants with different dress codes. My friends went to a resort last year with 3 kids, and no one dressed up, but they chose the more casual restaurants in the complex. So ask up front - they are used to such questions. But I would be surprised if this was a dressy affair like one of those "dress-for-dinner" cruise lines. Certainly you would have options.
You need 2 bathing suits per person (one to wear, one to be wet and drying off in the hotel), and a cover up for you. If the men wear longer "board shorts" type bathing suits, they might get by with a tee shirt over it to go from room to pool/beach, but if they wear skimpier speed-o type suits, they can't go through the lobby or the lunch buffet in those. You need something else. You might get by with a light dress or sarong skirt that can double for another wearing.
I think you always need a wrap/jacket at night because you have ocean breezes. You also need something on the plane. Take one thing that doesn't wrinkle and that can get you through a variety of situations, like a white sweater with a simple pattern that might look dressy enough over a long skirt.
Airport - you will be in a car for most of the trip, so wear jeans or even a pair of loose pants that you can wear leggings or yoga pants underneath. Wear a sweatshirt with 1-2 things under it if you can avoid wearing a winter coat too. It really depends on where you are parking. If you are leaving your car in an outdoor lot and have to shovel your way out when you return, you'll be miserable without a coat, gloves and boots. But if you are getting a ride or taking an airport shuttle, you can wear less and just double up some layers. Take a collapsible bag on the plane with you for ditching all that extra clothing en route or when you land.
I think separates in coordinating colors work the best (so you can layer or mix/match), and if you have fabrics than can be rinsed in the hotel room sink and hung up to dry, you will be very happy. Take a small bottle of something like Woolite and a few of those folding travel hangers or travel clothesline (unless it's a resort that includes a line in the shower - some hotels have those in the US too). I would think that capris would be very versatile. Plan on wearing things several times and/or splurging on the hotel laundry service if you're not a rinse-out-in-the-sink sort of person. If you will use the hotel laundry, take one of those lingerie bags (mesh with a zipper) for keeping socks and small items together.
Excursions? You NEED solid, sturdy walking shoes or excellent sneakers with a good tread on them. You are not climbing Mayan ruins in flip flops, sandals, Crocs or flimsy bargain store sneakers. You just aren't. It does not matter what the kids say is cool or not cool. It is not negotiable. Also socks (2 pairs per person - wear one pair, pack one pair). Nothing ruins Excursion #2 like blisters acquired in Excursion #1, with the added complaining! Take something simple to wear onto beaches, but wear a good pair of walking shoes on the plane (to save space in the suitcase) and then use them for your treks.
Have fun!