Dual cable brake levers work great on pairs of wheels (trikes) but not so well on bicycles. Because, as has been mentioned, they become very difficult to modulate. There's also the issue of pulling two brakes via one actual lever with tiny fingers. It's just too physically hard to do. Kids bike brakes being shite very often need both hands to operate at the best of time. And... Illegal, bikes need two separate brakes. What we often do is simply relocate one brake lever so that it sits under the other. Stepped so that the lever can be operated by separate fingers. But again, this requires strong fingers of the kind rarely found on five year olds. Coasters work perfectly well for most of the world's population, but for some reason they're rare in the UK. I blame Raleigh and Sturmey. They're strong, reliable and easy (once learned..) to operate. They can even be supplied with gears. SRAM still make the excellent and bomb-proof 3 spd Torpedo. And Shi**no make an equivalent. These can be built into any size wheel - but will require a frame with a matching OLND and horizontal drop-outs. Which all means that it's probably easier to buy a bike which comes equipped with a coaster from the factory. Even if that means having it shipped in from Holland or Germany. Check out some of the euro style kid's bikes from the likes of Puky and Gazelle. Unfortunately the importers usually don't bring in coaster equipped bikes. Because Brits don't know what they are. If you can make it to York I have loads of bikes with coasters she could try.
The other thing is, she'll need some work done to the handlebar.
You could call me if you like. Link in my sig. Ask for Mick.