According to my source, sanji hesitates because Zaza is a beautiful goddess but he can also hear the screams of the children and see them struggling. He's frozen in place for a moment and doesnt know what to do, but right then, he subconsciously looks into the future and sees the children drowning, some dead already...
Right before he obliterates Zaza with a massive explosive ifrit kick, we see a quick flashback of his conversation with gaban about conqueror's haki. Gaban gave him a complex answer... Lets wait for the brief spoilers but this is the sanji section of the chapter.