Maybe you can wait until you get there and decide then?
If bubs will be asleep by 7...could dinner be booked for 8 and then you put bubs to bed and then have someone sit with him? Maybe you could hire the same person to do something with you during the day so you can see how they interact first hand and then decide?
It is a tough one...it would depend on so many things for me...how far away the restaurant would be, if i had met the nanny, the sort of place I was staying and how secure I felt there, how long i would be gone, if bubs was a good sleeper (or was sleeping well since arriving on holidays).
At the end of the day, you are the mumma and it is up to you. It is only 1 dinner out of the holiday...