How much should a nanny be paid for watching 2 kids 5-6 days a week?

I'm a nanny watching 2 kids for a chief in the 
Navy. I work 5 days a week, sometimes 6, with an overnight I have to watch the kids because he has 24 hour duty. 
I have to be there at 5:40 in the morning Monday thru Friday, and I usually am here Til about 5:30-6 in the evening. I'm currently pregnant with my 1st. The kids are 7 and 12. 
I get paid $200 a week, not including the overnight I work. The weeks I work overnight I get an extra $50, so sometimes I get $250. 
Is that too low? Or Just right?

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