I have an idea for the stat bonuses on the transformation. Double the values, but they only apply in the opposite form (bonus AD/AP only applies in Frightened Child, armor/MR only in Violent Woman).
Another thing that I think would fit this would be the idea of the magic of the league allowing Syl to take more of a physical form, especially since, by her lore, Sylvia is actually 19. The way it would work would be that every point into Violent Woman (Also, I'm not a fan of that name. It just sounds too bland) would cause Sylvia's body to change. 3 points would make her into a full grown woman (I imagine something like Syndra or Zyra, just because they seem to have similar personalities), making it even clearer that Syl is gaining dominance.
Another idea for how to make going 2/1 or 1/2 on the form choices would be to give those results special bonuses beyond the 3rd form of the abilities. I don't know what the bonuses could be, but I think that would be the best way to encourage that choice.