Doesn't notice

Somehow good mom Toriel doesn't notice the thread (it is a long thread), and deciding on cleaning the house first she steps out of the room.

Not noticing frisk stepping out of his room aswell, yet he noticed her and her thread. Being curious he silently walks up behind her and pulls the thread, not noticing anything happen he firmly pulls it 22 times ( it is important to note that Toriel has stopped right outside her door to think about what to clean first).

This had caused the dress to end right under her crotch before ending in 3 threads, if Toriel stands in a certain way, squats, kneels, or move to quickly it would expose her crotch and a portion of her large butt.

Feeling bored Frisk leaves back to his room to nap, Toriel dexoding to clean the hallway first had summoned the cleaning epuipment from the pocket space that she had.

Getting down on her hands and knees to scrub the floor, this had the effect of exposing her butt and intimates to the empty hallway.