Kid thinks she is dead, like we do, because she/we lack/s of information. In the dream/flashback scene with Lynx/Harle and Serge we don't see any sign of Lucca, she didn't even get her Ice-Pistol to stop the fire, nor is there any corpse or something(If they killed her there, why would they hide her dead bosy? The house was going to burn anyways). She was probaby kidnapped, because she is not that kind of person that leaves her Children in a burning house.
Haven't played CC for a while, and I think we don't get any other information, the rest is just speculation on our part. Faxt is: She isn't there and Kid asumes her death->wants to take revenge.