Keepers of the Flame by Jennifer Armstrong

Five years have passed since a virus killed most of the world's adults. The children who have not died, scavenge for food and shelter in the ruins of towns and cities. They cling to their memories of the pr-virus (fire-us) world, but these memories are distant and distorted. A small group of teens and children are travelling up the coast of Florida and for the first time in five years they have come across adults. Living in a mall, these adults welcome the group warmly and invite them to stay. However, there is something peculiar about these adults....

While this is a trilogy, it was easy to jump into book two without reading the first book. It's a great seat-of-pants adventure with some real depth to the characters and their will to survive.