wait, hang on, are you paid for 5 days a week, but only work 3, or are you paid for only 3, with a clause that means you can't work elsewhere on the other 2, and now they want to pay you even less while effectively keeping you free for them fulltime hours?
They are taking the piss and however nice to your face, are not good employers.
If they want the flexibilty of a full time dedicated nanny, they need to pay for it. If they want to have the cheaper option of only paying for care they need, then they lose the flexibility and dedicated nature and have to start using a childminder.
You need to be a little selfish, they aren't going to be the only employer you have for the next 30 years, sooner or later, those children wont need a nanny anymore and you'll go to another family. Sadly, they aren't prepared to pay for your services anymore so it'll have to be sooner.
The only other solution that could work for you would be to train as a childminder yourself. Set up a business, but they will have to bring the children to you and collect them, they accept you'll have other children and your flexibilty will be based round when you have available places, if they want to hold 5 days a week they'll have to pay for 5 days a week, if they want you to only be working during before and after school, then fine, but you'll have other children and so you might not be available in the holidays or inset days.
But it might just be better for you to say no, see if they want to make you redundant and look for a new family to work for.