We're land based with stays at Peter Island (5x), Cooper Island (1x) and Virgin Gorda (2x), so I thought I'd throw in a few comments. We've loved all the places we've been in the BVIs, so you can't go wrong at any of the places you've mentioned.
Deadmans at Peter Island is a gorgeous beach. Its longer and more picturesque than Cooper. The restaurant on Deadmans at lunch doesn't require a dressy style at all. A coverup over a swimsuit is fine for women and men can wear a t-shirt and swimsuit or shorts. Sometimes we've seen groups of business people from Tortola eat lunch there and they are dressed up, but the resort guests and day trip visitors are usually very casual. We've always enjoyed the food there.
When you're in North Sound, you may want to consider going to Saba at Happy Hour and for dinner. If its the same as 2 years ago, they feed the huge tarpon at the dock and its quite a sight. Your daughters may enjoy it. We had a delicious dinner there.
If you rent a car on VG, be sure to drive up to Hog Heaven for a drink. It has a truly amazing view of the North Sound area and an easy drive. Its just stunning and one of our favorite views in the Caribbean.
Happy trip planning!
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