I've not done it myself, but know that others have taken children up and either do a booster type seat or a couple of pillows to boost them up so they can see.  They also put them in the front seat where the visibility was good and they could watch the instruments, and was a lesser chance of feeling clausterphobic, plus if things go well, you might let them fly a little bit.  I remember the first time I was given the chance to "drive" the plane.  I only did a few shallow turns, but it really gave me the itch.....of course I was a little older....maybe 14.  Your grandkids are probably much younger, but think they would still enjoy the chance just the same.