What is the blackfoot or siksika word for granddaughter?

Answer 1

Lakota for grandchildren is "takoja".
The Blackfoot are divided into the Siksika, Pikuni, Piegan, Kainai, and Blackfeet.
The Blackfoot language, wile the Cheyenne and Cree, is part of the Algonquian Language group.
I googled and found these:
http://www.native-languages.org/
http://www.native-languages.org/blackfoot.htm#language
http://www.native-languages.org/blackfoot_words.htm
http://www.angelfire.com/ar/waakomimm/language.html
http://www.angelfire.com/ar/waakomimm/lang13.html

These sites gave a smattering of words, pix, etc., and refer one to another place to buy a book, a dictionary.

Answer 2

I'd be interested in the correct answer too. The Siksika is but a family among the three main groups of the Blackfoot. The others are the Piegan and the Blood.

(Notice that Siksika and Blackfoot are proper nouns and ought to be capitalized.)

Oops . . . I had forgotten about the Siksika, Pikuni, Piegan, Kainai, and Blackfeet as the families of the Blackfoot--thanks to the response below me.