Question

In the City of Ember,  Why would it frighten the children to see Ember's current storerooms?

Answer 1

They were frightened because there is a haunting environment inside the storerooms that were left useless for many years. No one dares to step into the store room except the plumber and the messenger Lina. There was also quite darkness with a chance of power failure inside the storerooms that frightened the children. 

Answer 2

Ember's current storerooms were so dark and their was no natural light.
They children's were also frightened because they suspected that the lights may go off and the generator may also fail that would leave them helpless in the dark surroundings of the storerooms.