I‘lol go a different direction with my answer, based on some info you included: cruise month.
Next year, in August, 7-night cruises. Nothing is showing available for Dream. Is it in dry dock or sold out? Either way, you are left with Alaska on the Wonder, Northern Europe in the Magic, and Eastern or Western Caribbean on the Fantasy.
Alaska and Europe are almost always more expensive than the Caribbean. I don’t know how much airfare costs for Canadians or how flying time compares.
Alaska is one of my most repeated cruises. It typically involves 3 days in ports and one day cruising through a fjord to a glacier. A variety of port excursions are available, on which people do take their young children. It is easy to run up your bill with activities. This would be the wonder. A verandah is nice for one of the rooms for the beautiful scenery (Putting grandad and twins on the reservation for an inside - they don’t have to sleep together, though. Get a family stateroom with Verandah that sleeps 5 for the rest. (My money saving tip)
Europe is usually a port-intensive cruise, but port days are often historical-location centered. Lots of walking. This would be the Magic. I‘d save it for the future.
If you enjoy the beach /Caribbean, you have the Fantasy option. It will be similar to the Dream you will be on, so you should know if you enjoy that size/ class ship. Comparable kids’ clubs (pretty sure), pool deck layout, cabin size. The decision would be Mexico and the Western Caribbean islands or the Virgin Islands on the East.
It will be peak hurricane season at the end of August, but there’s no guarantee anything will happen during your vacation.
(For the record, I’m probably Magic > Wonder > Dream > Fantasy. And it is reflected directly in both # of cruises and total days on each ship. A lot of that is due to itineraries and price).