Well, if the nguzo saba were part of the original elaboration of the festival, then a seventh letter (third a) was, too. But if the nguzo saba were originally the nguzo sita (six principles), or if the whole notion of nguzo came along later, then the seven children myth could have historicity. I have inclined to the supposition that the final -aa was chosen to counteract the English-speaker's natural inclination to make a final -a schwa, i.e. a neutral vowel like the "oi" in "porpoise", instead of a full-bodied "ah" like the final vowel in Swahili "kwanza".

BTW I find it rather silly to call Swahili "Kiswahili" in English. Kiswahili is Swahili for "Swahili", but the "Ki-" is a Swahili prefix more or less equivalent to English "-ish" or "-ese" in "Swedish" or "Japanese". If we're not going ostentatiously to go around calling "Swedish" "svenska" (be sure not to capitalize it!) and Japanese "Nihon-go" (the -go being equivalent to Swahili Ki-), then why the insistence on Kiswahili? Occasionally you even see (in English) "isiZulu", where "Zulu" ought to suffice. I think major African languages are just as much entitled to their own English names as are major Asian or European ones.

(Sorry about the rather off-topical rant.)

Haruo