If you want to travel with kids, and to enjoy the Whale Watching, Valdes Peninsula in Patagonia Argentina is the point. You can see many whales, meters from the coast and the Whale Watching tours depart daily from Puerto Piramides, starting early in the morning and extending until the afternoon.
National Geographic Traveler Inteligent Travel wrote about this matter. Through the experience of Rainer Jenss who traveled around the world with his wife and two sons.
Any time is good for a natural event, where the protagonists are the whales.