I haven't yet reheated frozen croissants, but I like to crispen them up after bringing them back from the bakery, so I pop them into a 350F/175C oven for 5 minutes and they're done. If I had them frozen, I'd let them come to room temperature (shouldn't take too long in St. Martin) and pop them in the oven. Might take an extra minute or two.

Hmm, having croissants in the freezer sounds like an excellent idea.