RealFlight is the name. And you can get discounted prices on G3.5 right now because G4, their recently released improved version, has just hit the stores.

The developer has a popular forum that's got support as well as downloadable airfields and airplane paintjobs, and some standalone airplanes. http://www.knifeedge.com/forums/

But since you seem to wish to stay away from downloading stuff, the forums there would be useful to you primarily for chat and such. And they are quite useful for that.

AND since you wish to take the easy road, the RealFlight sim is going to be the only one that offers you a ton of airplanes and airfields that are available on CD. No problem adding new stuff. Just insert the CD and click on Install.

RealFlight comes out with a new CD every couple of months or so. They've got 5 CDs called AddOns that work with G3/G3.5 and 4 Expansion Packs that work with G3-G4. They average about 14 airplanes/helis a CD and 3 or 4 airfields too.

Keeping a grandchild interested sometimes takes new stuff every once in awhile. And those CDs are going to be about the only way you'll do that and avoid downloads. If you can insert the program CD and click on it's install, the ones that give the grandchild some new things to fly will be a piece of cake.

And RealFlight pumps them out regularly and seems to have a schedule for them. And no reason to be Frightened by a company that is healthy, produces in volume and on time.

As for which sim is the most accurate..... All of them. Every one of them has it's bigots and detractors. Some louder than others. But you won't go wrong with either of the most popular or any of the others.