Consider the following pokespe

did she think kids just appeared? she grew flowers there to keep break the fall and she checks there often

uj.....

the flower bed was for that reason, but it was a memorial for the first human and their falling.

aka Chara

Toriel thought the rest of the underground citezens/monsters were 'kill or be killed', so she closed off the doors to protect the children who fell.

plus, thats about a 500 foot drop from the surface, so for all the monsters to do that, it'd be hard.

asgore killed to free monsters, she could've freed monsters mercifully

but how?

i forgor what happens at the end btw when everyone leaves the underground

when everyone leaves the underground, war is averted
toriel could've took monsters to the hole where we fell from but she didn't