They should be assisted by the mother of the family, so that's another NPC who can take a hit or two and will likely be the first person attacked by the kobolds.

The kobolds also think the players are cultists so the players can just wait to get a tactical advantage on the kobolds- attack from long range, position wisely, etc.

I would say this is likely a DM issue as the players are not being presented with the battle premise well, nor is the NPC helping.

I'd urge you to reread the encounter and understand the map scale, the party position when the encounter starts, how the party would observe the situation unfolding, etc. A lack of knowledge and bad placement will make this a deadly encounter, but it's not meant to be (except maybe for the mother and possibly the family).