That is in agreement with my original statement - that I'd wait for additional material in TFU before making judgment, but at this point it seems cowardly. It's possible that the younglings were killed by Vader before Shaak Ti could have possibly gotten to them to help. At this point we don't know. But from what I read on wookieepedia it sounds like she just resigned and let the younglings be executed without offering any type of help whatsoever. Even the youngling that ran out on the landing pad when Bail Organa showed up gave a fight (I think that kid was related to Lucas somehow, if I remember right).
Anyway, I didn't mix up any canon, as you suggested. All I said is that it wasn't guaranteed she would die. Perhaps she could have defeated Vader and helped some younglings escape. She obviously knew some way out of there since she was able to escape herself.