Take a look at Cannondale Tandems, they build a tandem with a 16" stokers seat tube. I have had riders as short as 4' 8" on mine. You will need to use a longer stoker stem so she can reach.
Or you can use a kid back crank set, this is a crank that bolts to the seat tube of a larger tandem so kids can stoke. Take a look at Burley Tandems web page, very good tandems by the way.
or a full out custom...
"When the spirits are low, when the day appears dark, when work becomes monotonous, when hope hardly seems worth having, just mount a bicycle and go out for a spin down the road, without thought on anything but the ride you are taking." - Arthur Conan Doyle