If you are interested in your grandchildren's inheritance, I'm not sure bikes are the way to go. Start them a 401K and if we don't abandon our economic system by the time they can benefit, great.
If you are interested in your grandchildren riding great bikes that hold their value and don't break the bank, there are great Japanese bikes all over the place.
The two goals don't necessarily complement each other. I'm not sure even nice Italian bikes are an investment to leave as an inheritance, other than a cool bike.
Now, if you want them to have ardent admirers:
