Board Thread:General Discussion/@comment-15494492-20160423211921/@comment-27620479-20160424075537

I don't think she wants to do that. Not even in Frisk's case. She asked Sans to protect a human, and it is heavily implied to be Frisk. Sans believes that they are, and there are other clues to support it. So, Toriel seems to have talked to him after meeting Frisk, but instead of taking the opportunity to destroy the entrance while the human was supposedly waiting in a safe place, she just asked Sans to protect them if they leave.

When you unlock her talking about destroying it, you can can take Frisk back to their room (as she asked to), and try to have them sleep. Or you can have them run away while at 2HP, which would supposedly mean you have learned their lesson, and make sure they will be a good (even if terrified) child and stay with her to hear more snail facts. Or combine the two, just to make sure she understands that you understand. I think you could also try to wait things out in real time, and give her hours to do her business, although I didn't try that.

Destroying the entrance would probably keep the humans inside, but permanently cut Toriel off from the rest of the underground. And I don't mean joking with Sans, that would probably be still possible, or go shopping, which she can definitely do. I mean willingly choosing eternal exile. Toriel may not want to do that, and she doesn't seem to believe in restricting others' freedom above a certain level. She will even willingly sacrifice herself if Frisk fights for their freedom too eagerly.