yep I think you would need it to be someone you and the family had been around for a considerable length of time... 2 weeks is a long time so you would want to know how the nanny would cope and also how the kids would be with her/him.
Maybe you could start with a weekend soon and see how that goes?

I think having all the kids there would help - they would get comfort etc from each other

To be honest, at this stage I couldnt do it for 2 weeks (the mosyt i have left the boys so far is 2 nights w DH and one with my mum) - but MAYBE it could work for you.