Either Dragon didnt know, or he didnt trust Robin to be able to keep Saul being alive a secret, or Saul being alive was a retcon.
Dragon was in Ohara with Vegapunk while the giants were taking the book,and if i remember well,Saul was there too. I think Dragon just dont know Saul personally,and doesnt know about the bond between him and Robin? By the way yeah Saul could have helped her during her childhood,what the hell! And his survival may be a retcon.
By the way so far has Robin's time with the Revs during the timeskip been useful to something? It just created plot holes like this one or Robin not telling Luffy about Sabo (altough its justified with "Sabo had to tell him personally",but even there,Oda could have foreshadowed this during the reunion in Sabaody,it would have created a mystery like he always like to do).
It would have been better to make Robin spend the timeskip as an archeologist Lara Croft style