I have been lucky to have worked for some wonderful families, who value the work of the person who is responsible for their children, and who pay accordingly.
Unfortunately, there are also quite a few families who want to pay less than the rate you explicitly have stated (which is a very reasonable rate), or ask long hours with a very low weekly rate, or demand things beyond what you are paid for, even if you already go above and beyond for them.
I have gotten quite good at knowing which families are going to be good, but when you're in a slump, sometimes you are willing to put up with worse in order to pay the bills. I deal with the same thing being a musician. I often have to discuss with myself about whether to settle for a low offer, whether to negotiate, whether to refuse anything below what I have asked for, whether to just put up with it because I need the cash. I still don't always know what the answer is.
What I do know, however, is that it is very important to value the people who are offering a service: people like child carers, cleaners, nurses, teachers, bus drivers - the daily servers who often go unnoticed, underappreciated, and underpaid. Also young people, who are working hard and building up experience in whatever field they may be in. Please do not short change them. Their work is of no lesser value than yours, and if you want a high level of service, you need to make it worth their while.