Heck, I can make that drive in my sleep, being from Buffalo and having lived in the DC area for 3+ years. Lemesee... get yourself to Buffalo, then take I90 East to I390 East/South (you'll be at Rochester and be headed away from the city). Follow this until you get to just outside Corning - it'll have turned into Rte. 11/15 at that point IIRC - take Rte. 11/15 South through Pennsylvania and Maryland until you get to the DC Beltway - from there you're on your own. If the directions from Yahoo! differ significantly try Mapquest.
As far as time, it's 7 hours door-to-door from my house near Buffalo to my Sister-in-law in the Leesburg, VA area, about 1/2 hr. outside DC, including 1 reasonable meal stop and one other short break. Add on the trip from TO and you're probably looking at 8+ hrs.
Enjoy the ride. There's some nice scenery and lots of porno shops(!) in rural PA but that's about all there is. Downtown Corning is about 10 minutes off the trip and a nice little place to stop and walk around if you or the kids need a break about then. If you're a baseball fan you pass right by the Little League hall of fame and museum in Williamsport, where the Little League World Series is held every year. You can also eat surprisingly well in the Wegmans just off the highway there - everything from subs to sushi. Also, regardless of what the map program says don't take the bypass to stay on the 4-lane highway at Williamsport but stay on 15 (this will take you past the Little League stuff, which will be on your left opposite McDonalds about 1/4 mile past the bridge). The bypass costs about 15 minutes but avoids one pretty steep mountain climb and decent - your call. Don't speed too much in PA - the cops are pretty ruthless and use VASCAR not RADAR so are very hard to spot. Also, gas is cheaper the further you get from Buffalo and significantly so in PA so you may want to plan accordingly.
Scott