ETA
In regular child care or where you have a spot reserved for your child you pay in advance. This is why you sign a contract before your child starts attending. For instance, in my child care center, I had that the payment was due on Monday. If it wasn't paid by the end of the day on Monday your child was not allowed to enter class on Tuesday morning. I had a waiting list and could fill that child's spot within hours.
I had to pay my staff regardless of attendance. If kids were out sick, if parents took their kids on vacation, if they were on school holiday, etc...my staff was still there. The lights were on, the heat or air was on, their food was still purchased because there were supposed to be there.
Just because a parent kept their child home did not influence if my staff got paid of not. My staff got paid for being at work.
When you think about a nanny it is the other way around. You do not pay her by the hour or day or month whether she shows up or not. Her presence is required each time. So you pay her as an employee. Regular child care is nothing like paying an employee. Paying a nanny is like paying any other employee. Either it's contract labor and, non-tax on your part, where you pay her $X per hour or your terms with no benefits like you hire a plumber to come in and do a specific job or you hire her for a taxable job where you withhold her stated and federal taxes and send her taxes and Social Security in with a matching part by yourself. You might allow her to accrue vacation days, sick days, retirement, etc...if you do this sort of contract with her.
When I was a nanny I worked for a family with 7 children. I wasn't responsible for more than 1-2 during the day but when they came home from school I had snacks ready for them and got them going on chores. When their dad came home about half an hour later he checked on chores and got them starting dinner.
I was paid strictly by the hour by the child. If I had sick kids home from school I got paid for an extra kid all day. If it was a school vacation day I got paid for each kiddo under the age of 10. They had a teen daughter that was more than capable of watching the kids but they wanted someone there that was dedicated to the younger kids, toddler through elementary school.
I did not have taxes withheld, I was contract labor. I got paid by the hour by the child and nothing else. I didn't work summers or school breaks. They claimed my pay on their income taxes as child care. I claim all my income so I'm not worried about any of that.
I also had her bosses child join the group. The boss paid me by the hour but she held out taxes and paid into my social security. They also claimed child care expenses on their taxes.
Having a nanny is not like child care or pre-school or anything. It's contract labor that you hire to come into your home and do a specific job. It's up to you as to how you manage the pay and taxes and such. You need to sit down with your spouse and decide what you want to do. You can let her know she can expect consequences if she asks again OR you could ask her if being paid weekly instead of whatever you're doing would work better.
Pay her every Friday at the close of the day. For her time that week. Or hold it back and pay her on the next Monday.
Emergencies happen, that's a given. If you work in a company that has a credit union you can go in on your lunch break and get a signature loan or draw money from a savings account. In a home situation like this there are no alternatives to asking the boss for an advance.
Consider what her needs are, maybe she's too intimidated to tell you that she isn't making it from payday to payday.
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When that happened to me at my child child care center I asked them to come to my office. I simply told the person that I would do this one last time and then they would be terminated if they asked again. I had told them I wasn't even taking money for my own household, "I" made sure my staff got paid first, then the utilities were paid.
If you don't want to test her loyalty you could tell her that you will do it one last time and then you are moving her paydays to once per month and if she approaches you again you will hold her pay for one month. She will work a month then that pay is held to make sure her hours and any charges are taken care of. Then she is paid the next month. How many times have you worked at a business that ends payroll then it takes a couple of weeks for that pay period to generate checks? I've worked at several places where I worked a full month before I got 2 weeks pay.
For her to come again and again, asking for money when it's not payday...and you've let her know you don't like it, that's just "oh well, she'll give it to me anyway".