From the information you are saying, I understand that you and your daughters have been suffering from diarrhea
from a wk roughly. I agree with you regarding the less likely nature of an infection to stay for a wk. But as the infectious diarrhea is the most common one, you got to be treated first as if it is infectious. The most common reason for the infectious diarrhea is viral and most of the time, it enters the family through younger children as in your case. As it is a viral one, the virus got to be flushed out of your system to get better. So, no medications for the diarrhea are recommended. The most important factor is that as you lose lots of water and fluid in the diarrhea, you have to replace them by hydrating yourself appropriately. If it is the viral infection, it will get better on its own over the course of time. I agree that 1 wk is a little too long but can happen. The next one to look into is are you getting constant food poisoning
through some thing in your food that you all are getting exposed. I wounld recommend to relook into the various things that you are eating like cheese, margarine, etc if they are approaching the expiration date. sometimes, without our knowledge, they can get fungus or virus upon improper storage. I would recommend to look into all the sources of infection that can go into your system orally. The lactose intolerance
is also another factor to be looked into but it is not very likely to start all of a sudden in the entire family at the same time. Once you look into all these options, if you still could not find the source, please consult your primary care
physician and get some advice. Whatever may be the cause, the most important thing is to keep yourself hydrated with fluids, electrolytes like gatorade in some situations.
Hope this helps you to some extent.