BWI to Hershey: figure about a 2 to 2.5 hour drive.
Philly to Hershey: again figure about a 2 to 2.5 hour drive.
Both could be worse if you hit the traffic wrong. Philly has the potential to be MUCH worse. I live in York County, about 100 miles from Center City Philly, but have to go to Philly regularly for work, and in rush hour it's not unusual for it to be a 3.5 hour commute. What should be two hours turns into three and a half just because of traffic.
So that said... I'd do BWI over Philly in a heartbeat, if you're going to pick a major airport.
BUT.. The most convenient thing is absolutely to fly into Harrisburg (MDT). Very very close to Hershey.
Three days is enough for the area. People keep talking about Dutch Wonderland, but my daughter has been going to Hershey Park every year since she was a toddler and always loved it. (She's eight now.) We did Dutch Wonderland once when she was younger... maybe about four years old?.. and although she enjoyed it, she prefers Hershey Park.
Yes, Hershey is known for the coasters, but there are lots of family rides, too. The Ferris Wheel, the log flume (Coal Cracker), the monorail, the train, the carrousel, tilt-a-whirl, skyview.... And as previous posters mentioned, there are kid rides scattered in every area of the park.
As for the coasters, there are two very tame ones, the Trailblazer (barely even qualifies as a coaster, less thrilling than Big Thunder) and the Super Dooper Looper (it has one upside down loop but it's pretty low speed, my daughter went on it at age 6 and loved it, and that's a kid who wouldn't even ride Expedition Everest at age 8).
The Boardwalk area (water park area) opens May 26 so you'll miss that I guess...
Indian Echo Caverns is nearby, also, if you're looking for more. Haven't been since I was a kid but remember it being enjoyable, and my daughter went with a school group last year and loved it. Google it if you're interested, it'll come right up.