Just take your time and both get used to it, it's really not that bad.
I took the missus out for the first time last year and it was ok.
I figured I did plenty of hooning around with 2up on field bikes many years ago so the roads should be easy.
Our trip was from St.Helens to Bakewell near Matlock for a dirty nice weekend away, it was pitch black and quite foggy by the time I got anywhere near and I was (still am) unfamiliar with the roads around there so was going really quite slow....there was a landrover with a trailer on trying to get past us at one point.
On a side note - Just as we was about to set off the mother in law said, "I don't know how you can both get on that bike, if something bad happens your kids have no parents"
Well what a lovely thing to say, true I suppose but not what you want to hear.