Do you mean TFCON in Mississauga (just outside Toronto proper)? If so the most central location for an AirBNB may actually be in Mississauga itself. Even though it is geographically close to downtown Toronto, it takes roughly the same time to drive to Niagara Falls and downtown Toronto, due to the heavy traffic during the week on the QEW and 400-series highways.
To minimize the time stuck in traffic, have you considered booking different AirBNBs throughout your stay here?
I recommend the following:
1. Having an AirBNB in downtown Toronto near a subway station. You can buy a city pass for the ROM, CN Tower, Ripley's Aquarium, Casa Loma and Science Centre/Metro Zoo:
https://www.citypass.com/toronto
You can easily spend a day each at the ROM, Ripley's Aquarium and the Science Centre / Metro Zoo. CN Tower and Casa Loma are each half-day things. I'd add the Eaton Centre too if your family wants to do some malling.
With the exception of Metro Zoo and the Science Centre, the rest of the attractions are in downtown Toronto and reachable by public transit (TTC), which beats being stuck in traffic or spending time (and significant money) finding parking. TTC is free for kids 12 and under.
The Science Centre is reachable by TTC, but that one is probably better driving.
The Metro Zoo is at eastern border of Metropolitan Toronto near the border of Scarborough and Pickering and requires driving.
2. Having an AirBNB in the Niagara Falls area. This will provide access to both the famous wineries in Niagara-on-the-Lake, Niagara Falls, Marine Land, Butterfly Conservatory, and the kinda family friendly attractions in the city of Niagara Falls itself. When driving in that area, please be careful you don't accidentally end up in duty-free / border zones, as it's a huge hassle to get out of those without ID / passports.
3. (Optional) AirBNB in the Mississauga area near the TFCON. This is only necessary, I think, if you intend to attend the entire 3 days. The convention centre is reachable from downtown Toronto if you don't mind the traffic.
Hope this helps.
