Dipper and Bill had tried to keep her away from this stuff. Bill knew the effect power had on him, and he doubted his daughter would be any different if she learned how to use it.
At this age, though, she had a tendency to wander away. They couldn't exactly stop her from heading into the woods, or exploring the town more.
And they had no idea when an old "friend" started talking to her after school. Pyronica had started filling her head with rebellious ideas, and they were none the wiser.