What is the point of this question? What do you want to be told, really?
It is not to ask whether paying cash with no tax and no NI is legal - you are well aware it is not.
Is it to ease your conscience? Would you feel less guilty if you were told that, I don't know, 75% of families pay their nannies in cash? What is the threshold that gets rid of your sense of guilt? Would you feel guilty if that percentage were 10%?
Is it to try and estimate the odds of getting caught? Do you want to hear lots of replies along the lines of "we have always done it and never got caught, so don't worry"?
Is it that you want to be told "do what you have to, you did everything by the book when you could afford it, so don't fee bad if now you can't"?