So your great-great-great-grandparents were tryndeez, or else really
were illiterate and spelled
Sadie Saydee just because they didn't know better. Neither is a good justification for continuing their mistake imo. To cut down on confusion, frustration, and your kid being mistaken for coming from a poorly educated family (the old "the apple doesn't fall far from the tree" thought applies here), I
strongly recommend spelling the name
Sadie.
Now, as for the name
Sadie itself, I admit I personally don't really like it because it reminds me of the word "sadism". But it might be just me, so take that as you will. I also prefer
Sadie as a nickname for
Sarah, because I generally dislike nicknames or pet names as full first names, unless they can etymologically stand on their own. Also, I feel a full name gives the child more choices, because--in this case--if she winds up not liking
Sadie, she can go by
Sarah,
Sally /
Sallie, or
Sal, or even a more obscure nickname like
Sari,
Sassa,
Sarit,
Sarita,
Sarina, or
Tzeitel.