The St. Lawrence, Great Lakes, and Lake Champlain have some of the best diving in the world, less than a tank of gas from your home, and Caribbean DMs just about drool when I tell them that I regularly dive on wrecks from the 1800s, and that some locations are so clean that the water is drinkable.
My suggestion would be to save the money from your trip, wait until next July, get them a nice long, local OW class with checkout dives in the St. Lawrence. The water should be in the high 70's by then. Before the water starts cooling down, your kids can do more local diving than they'll get in decades of "Discover SCUBA"
And once they get their own equipment, a local dive costs the price of an air-fill (generally in the $6-$12 range for air).
Terry