I think the baby voice is because she wants people to see her as innocent, simply "called" to the task of being a cult leader, and that she would never think of deceiving < baby voice > anny-wun < / baby voice >.
And I think it works on some people - makes them feel protective of her. I mean, note the name she chose for herself: "Little" combined with the preposterous "Grandmother" (yes I know it's possible to have a grandchild by the time you're thirty, but, really.) Those are words that attempt to broadcast, "I'm harmless, I couldn't hurt anyone, I need to be protected." [cue the tears]
Whereas many of the male shameons and cult leaders go for the big, macho, obvious power trip, her schtick is more insidious. I think it's designed to make people underestimate her while she empties their pockets and messes with their heads.