I'm going to ask my roommate to pay for a new laptop after her niece smashed mine.
Do you think your roommate will actually pay for it?
I know she feels guilty for it
It sounds like this wasn't deliberate, but if your roommate feels guilty, I hope that she is indeed willing to pay for what you lost. People should be willing to pay for anything their family members damage, even if it happens by accident. I hope you reach a reasonable solution to all of this.
She's going to throw a tantrum and make a huge deal about it.
Well, as you say, your roommate feels appropriately guilty, even if she is acting upset at the moment. I hope that she realizes that it is her responsibility to pay for the damage caused, and also that you be reasonable in only asking her to replace what was lost, and not to upgrade it.