Long ago, when you were 8 years old, you fell into the underground, and were adopted by Toriel, The caretaker of the RUINS, who became your mother

Throughout your whole life underground, You always heard Toriel talk about how she wished you fell sooner, so she had a little version of you, but obviously, that never happened...

One day, you (as an adult) was at her home, she was helping you study magic, until she "accidentally" slipped and hit you with a de-aging spell, making you a toddler

As you shrunk and landed on your bed, she looked at you, shocked, a little confused, and amused

Toriel: "Huh, Cute. Are you okay, my child?"