User blog comment:12th Supernova/Chapter 793: Bad News and Bad Translations/@comment-4503935-20150709200130/@comment-2125188-20150711103256

Because Sugar is arguably Doflamingo's most important member after himself. I'm sure Doflamingo would've done everything in his power to make sure no one finds about Sugar and/or her powers. Doflamingo has a very large crew, and that it would be hard to know every single one of them. Also, Doflamingo isn't always with a lot of members of his crew. I mean the majority of the Donquixote Pirates made their debuts in the Pirate Alliance Saga. Simply put he always has a lot crew members that are not with him, so it would very difficult for the WG to know who is or isn't with him all the time. I personally believe that Sugar is the one crew member who he always keep hidden in Dressrosa, and anyone other than the Donquixote Pirates who see's her would be turned to toys. Even if people know she exist and know her age, I'm sure Doflamingo would make sure no one knows her power, other than his crew and Sugar's victims.