First, choose which side of Niagara Falls you want to stay, USA or Canada? We chose Canada because its more entertaining. Are you traveling by plane or car? Bring enough entertainment if you are driving with kids. Also, be aware that kids need to have passports as well.
Aside from walking along the Niagara River, there is so much more you can do here. There are plenty of places to spend the day: museums, restaurants, stores, sport adventures and many tourist traps as well.
Any time is a good time, but I highly encourage seeing the falls during the day and at night as well. The light show displayed on the falls makes it look surreal. They also have fireworks at night.
Canada will definitely be better during summer time, with a fresh breeze, but also a sea of people. November is a good month to avoid the crowd, but be prepared to face the cold.
Traveling with a family of five can become expensive really quickly. I found there are not many Airbnb's near Niagara Falls on the Canadian side. If you are renting a car as well, do a bundle and save a couple of bucks.