Timing Will Be Everything
Ideally (all things being equal) overnight stops following a basic I-75 route from Orlando to Toronto would occur just north of Atlanta GA and near Lima OH. But seldom are all things equal. In particular your first day will require some forethought and planning. If you are starting your drive home on a Saturday, you shouldn't have too many problems. On a Sunday, however, you'd be in the midst of all the Atlantans returning home from their holidays; this would probably be your worst option. On any weekday, you would want to time your arrival in the Atlanta area for either before 2:30 or so, or after 7:00 so as to avoid the worst of the evening rush hour. Since arriving after 7:00 is far more likely, that means you can plan on a couple of stops of an hour or so along the way. The two that I would recommend would be visits to the west side of Okefenokee by way of Fargo GA, GA-117 and the West Entrance, and to Andersonville National Historic Site in the Georgia town of the same name. You want to make sure that you get through Atlanta before bedding down for the night so that you don't have to drive through it during the following day's morning rush hour.
Your next two days would be only a little easier with similar problems. On Day 2 you'll have to time your driving to miss rush hours in Lexington KY and/or Cincinnati OH. Possible stops would include Norris Dam State Park in Tennessee, Levi Jackson Wilderness Road State Park in Kentucky (hint: It's not a wilderness); or even the U.S. Air Force Museum in Dayton. On Day 3 Detroit will be your major headache, but you should be going through there around mid-day. If you have plenty the time, consider a visit to FGreenfield Village in Dearborn.
AZBuck
Not Nearly as Far as You Think
By the time you actually drag the kids out of Universal Studios, get the car set up for the long drive ahead, account for the fact that you'll have to feed everybody at some point, and realize that you're not as fresh as you were at the start of your travels, I don't think you'll get anywhere near five ours down the road on that first evening. I really wouldn't plan on much more than three.
Now, I thought you were planning on an I-75 return to Toronto. If so, that would put you around Valdosta GA if you just head up I-75, or Lake City FL if you plan to visit Okefenokee National Wildlife Refuge the next morning. In either case, and whether you visit the swamp, that would put you in decent shape to get past Atlanta the next day before evening rush hour. If it were me, I'd probably use the I-285 beltway around the south/west rather than go right through the heart of Atlanta on I-75.
On the other hand if, as it seems, you're now planning on a more hurried I-95/I-77/I-79 return, then just beyond Jacksonville is as far as you should plan on. That would put you just within two long, but doable, days of Toronto without turning everyone in the car into a cranky, unbearable companion.
AZBuck
The larger the party the more time needed.
Thanks for the explanation. However, it creates an other issue, which you will need to take into account. The larger the party the slower the trip. Every stop will take just that bit longer, there will be more distraction and activity going on in the van and chances are you will be stopping more frequently. It will probably pay you to still be on the conservative side that first day.
Lifey