Niagara Falls is quite a long drive for you for only one night. If your DS is a good traveller and doesn't mind being in the car for long periods it would be okay, but otherwise I think I'd go for London. There are lots of things to see and do there- a great children's museum, Storybook Gardens, art gallery, Fanshaw pioneer village, beautiful parks and walking trails and more. If I was looking for a place to take a 4 year old, I would choose London over Niagara Falls.
Maybe you could take a trip to the falls during the Festival of Lights they have starting the end of November. The kick off weekend is usually very Disney oriented-Mickey flips the switch to turn on all the lights, there is a Disney production in the park, Character meals, etc. The Falls are very beautiful in the winter when they are frozen and the lights are on them.
Whatever you decide to do, I'm sure you'll have a great time.
