Can I make a suggestion regarding the reward chart?

Take the money out of the equation, break it down into small periods of time, a month is a long time for 5 children to behave, a day lets them see instant benefits, and stick with it even if their behaviour gets worse at first. Make rewards small but meaningful things, like playing a game together or a trip to the park. So far they've had the message that if they push the limits you'll back down and try something else so why should they bother behaving? Let them push you and discover you mean business! Have the day out at the end of the month regardless if you want but don't use it as a reward, use it as family time.

Also stop fixing up things that get damaged else they won't learn to respect their home if good old mum/dad always come along and repair it. If its yours they work to pay for a replacement, if its their's tough, their loss. A bit of tough love won't do them any harm.

And ignore nanny...your the boss not her, if she says yes to something you've said no to the answer is still no. But remember nannies are supposed to do that, its in there job description to be a soft touch!