I find there is a 2 week itch, it happens around the 10 day point, though can happen earlier. I think it is once the realities of the job have hit home, you start thinking that you can't do it, don't want to do it, don't like it.
A parent working from home should have been discussed. Once you found out, did you then discuss how things would work so that you didn't get under each others feet too much?
Was food not discussed either?
It's not stupid at all, it sounds to me as though things did not get discussed prior to starting the job and still probably remain not talked about.
Are the parents new to having a nanny? Are they perhaps unsure about how things should work and therefore are not realising that it's little things that can make or break a job. Food on the grand scheme of things is quite small but can quickly become a major issue, if someone is working a 12 hour day without any breaks then in exchange for no breaks nanny often gets to eat with the children, eating the same as the children where practical. Whilst the parents may feel you should bring your own food to work and you may feel the same, the relationship between you and the parents needs to be such that you can talk about things without getting each others back up, not coming across as talking down to someone.
Can things change? If you don't feel able to discuss things with them, can you write to them/email? Not ideal but I've done that before... air the issues and propose solutions. Ultimately the parents want you to care for their children, they don't want you to leave. It may have taken them a long time to find a nanny, so they may not want to lose you but unless they know about issues then they can't fix them.