to add to what fred said make sure that if you cement the floor that the dirt is PERFECTLY flat or else the cement will be uneven and might even begin to crack over time
it will definitely crack over time, no doubt about it. i suspect the area that they want to use has very fertile soil, being a farm and all, meaning lots of roots. eventually the decay of roots, shifts in the soil, and underground life will break the cement. either way, cement would basically only me aesthetic, it's not like they are making a home, you just hide there from bullets. dirt floors would be equally as effective, except when wet.