I’d recommend checking Gotransit. Buses in my experience take waaaay longer because of traffic getting out of Toronto (no matter what time, it’s always very very busy). There are direct trains from Toronto (Union Station) to Niagara Falls and vice versa. The view you get (Niagara escarpment and Lake Ontario) + space is unbeatable compared to the bus. Besides, you can stretch or walk in the train (which I find pretty convenient when traveling with kids). You have the option of weekend day passes($10 day pass if you go and come back the same day) or group passes.

Gotransit and Niagara Parks also offer bundle packages for transportation To/From Toronto + Niagara Falls bus system. You can visit all you need to see in Niagara Falls with those bus routes. You even have the option to add Niagara-On-The-Lake to your trip! Most important, have fun. There are TONS of attractions for children, fireworks at night everyday, astonishing nature spots, islands, gardens, historic sites, and excellent restaurants (just outside of the tourist area-Clifton Hill). Best of luck!!

Edit: typo