There was a time when I drove in Downtown Toronto a lot. I had my parking spots scouted out, I knew my one way streets. I knew where I was going, and the best streets to take to get there to avoid excessive traffic or obstacles.
That being said, driving in Downtown Toronto can be quite STRESSFUL to even a seasoned driver. So if you are planning a trip from Guelph to Toronto to go to the ROM, or Ripley's Aquarium, or the CN Tower, you might just want to take a moment to think about how exactly you should get there. What mode of transportation is right for you?!
Are you taking the kids? How old are they? Are you bringing strollers? How long will you be staying? These are all questions you should consider when assessing the best transport mode for you.
DRIVING
The big advantage of driving right into the city is that you will have your vehicle there at the end of the day. Your kids can nap all the way home comfortably. You can leave things in the car and come back to get them if you need to (e.g. extra clothing, cooler with food, etc.) You can turn the air conditioning on, and you will be guaranteed a seat! If you are travelling during rush hour, the TTC can be packed with people. When you are travelling with kids and strollers, this can be less than ideal.
Finding a parking lot can be difficult, and parking is rarely cheap. What you might save on is convenience and time...well, you might only consider it convenient if you don't mind driving in downtown city situations.
DRIVING + GO TRAIN + TTC
Drive to Oakville and take the GO Train. The ride is about 45 minutes to downtown, and it goes directly downtown to Union Station. Tickets aren't cheap though. About $17 for an adult, $7 for a child. The trains are relatively frequent. The advantage is that you will probably avoid the bulk of the traffic along Highway 401, and most definitely avoid traffic downtown.
DRIVING + TTC
You can park at a few TTC stations (such as Yorkdale, Downsview, Wilson) and take the subway the rest of the way into downtown. Parking at some of these stations does cost you, but it is likely cheaper than paying for parking at a lot downtown. You lose some of the convenience factor of having your car nearby in a parking lot downtown. But you at least have it for the drive home, and for when the kids might zonk out more comfortably.