They were not carnivores before the flood.
We don't know how long after the flood it took for them to become carnivores.
We also don't know how many years' worth of food stores were on the ark or whether there was a garden that could be transplanted.
Granted, repopulating the world from a few survivors is a daunting task, but it is made easier if it is properly planned for. And they had the universe's greatest planner.
Shalom.