@SpaceFootballKing The servant thing could be (potentially) explained. Seeing how they're all religious zealots, fanatics and morons, it stands to reason the servants ALSO turned on the family.
Or, even if they didn't, that the parents didn't trust them.
The mom wanting to stay with her husband also makes sense. I'd say the choice of that, or leaving with the kid, may be 50/50. We don't get more intel on whether they actually were planning to get away or just stay and die.
As for the girl, well, perception in manga is weird (and, well, age is too, with kids depicted way too smart). I've been around a few pre-schoolers. While size-wise she seems small to be even 6, from the few lines she delivers she does NOT sound 5, let alone 4. I'd say 6, if you force me to give an age.
And it wouldn't be the first 6yo to survive on the streets...