Update. When I turned my game back on, 3 out of 4 of my family were a bit sick, so I did what I had to do and they all recovered fine.

The two kids who were extremely weak & a bit weak, well the extremely weak is now a bit weak & the other kid has a very high health bar. So I think it may have been the forgetting to top up the food the first night of the game. They seem to be improving themselves, just slowly.