If there is a strike how will it affect your numbers to have the 2 extra kids all day?
I don't think it that big a job. Its Monday-Saturday morning so you still get your 2 weekends. Ideally the kids are both in school so it won't be 24/7 for the full time. I think $100 a night is pretty reasonable. Could do more if you want. I don't think I'd charge for each child. It isn't really that much more work to have 2 versus 1...it may actually be easier as they can play together and keep each other company.

you've been working with the family for 4 years so you likely have a good idea of how the week will play out.