You guys are cookin'!!! Way to go. Hope you hear back from Find a Grave Debbie!
Yes, the forum is in consensus that "Sada" is your Sadie.
It sounds as if you knew that a Henry Senesac signed Sadie's death certificate. So that is further evidence this is the right family in 1900. Henry is likely John's Senesac's son, Sadie's brother - her only full sibling that we know of.
Martha's age should say 12, according to Taryn's post.
John Senasac and Sarah Gray (Crisp) married 01 Mar 1900, per tester's finding of the marriage record. That's just a few months before the 1900 census was taken in June. Thus Sadie was about age 11 at the time.
Loonyville, Nacogdoches, Texas
John V Senesack 53, born Sep 1846 in Canada [French Canada], to USA 1855 [age 9], married 11 years/ married 1889 [this is a mistake], Father born France, Canada; mother born Canada, France
Sarah Senesack 53
Sada Senesack 11 born Sep 1888 in LA [prob Minnesota]
Henry Senesack 9 born LA [should prob be Minnesota]
Octavia Crisp 14
Martha D Crisp 12
Julia Crisp 9
It is normal for a census transcription to contain some errors and discrepancies.