Nanny pay for 3 kids question

Long story short... my current nanny watches my 3 kids 7,6,3 for 15.50/hr with a lot of flexibility in her hours bc I work from home currently bc I’m pregnant. She is also helping my 7 and 6 year old with virtual learning. Well they go back to school next week and get on the bus before she arrives and get off the bus after she has already left. So she will only have my 3 year old who goes to preschool 2 half days a week (9am-12pm) and then she naps at 1:30pm until she leaves. So basically she will only have my 3 year old 3 days a week.

I’m due with twins in December. Our nanny wants to stay with us and we are trying to work out a fair arrangement when the twins come. We will be offering 17/hr for her to have the twins and my 3 year old. She won’t have my older two since they’ll be in school and during the summer we’ll be getting a college student to help out with the older children.

My predicament is that my nanny needs to make a certain amount of money to stay with us so we have to come to an arrangement of what she will be paid while she is just watching my 3 year old before the twins come and when she is part time during my maternity leave probably watching all 3 older children.

Has anyone been through this? What did you do to make sure it was fair all around and not lose your nanny?

Thanks ladies!