In terms of "difficulty" in using each of the variations of Pis, there's no difference. They are all basically the same, with the main difference being how powerful they are and how many GPIO pins they have.
I would suggest looking at our setup kits to get them started - http://www.modmypi.com/raspberry-pi/set ... del-b-kits
Depending on what you already have around the house will depend on which kit you might want.
If the grandchildren are interested in a bit of electronics and coding, then I can recommend looking at the "Hacker Kit