how much would be fair to charge?

Mo • Married 9/24/16. Mommy 7/22/17 ❤️
My husband was telling me this morning that 2 of his co-workers are in need of child care. I am currently a nanny but they will be needing me less in the fall and it won't be enough to pay bills so I'm considering leaving and/or getting a second job but that would hinder my ability to care for my own child who is due any day now! 
The one co-worker also has a pregnant wife who is 4.5 months right now and the other has a 6 month old boy that needs child care. 
My husband joked that I should watch them out of our house (yes I know I need insurance and permits and licenses) and his co-workers loved the idea. We're all good friends and spent time together before we all started having kids- you know how it goes 🤷🏻‍♀️
But I'm considering it as it would allow me to stay home with my child and make money. But 2 newborns and an infant is a lot. The one friend said he spends about 700$ a month on childcare and thinks that's an insane amount but when I worked full time as a nanny I made that in a week so I'm not exactly sure what would be a fair price to charge if I did end up doing this! 
I'd be feeding 2 extra children and having them in my home all day which honestly could be easy seeing as how most children at hay age sleep a lot and my husband and his co-workers work from 2am-12pm ish so I'm not exactly sure how long these kids would even be here - depending on when their wives work? 
Idk I'm just trying to figure out how much I could charge and if it would be enough