Perhaps you could dilute the juice with water, to get more fluids into her?

Aside from the diaper count, does she have any symptoms of actual dehydration? Does she make tears when she cries? Does her skin bounce right back when you gently pinch it away from the back of her hand?

If she is perky and active, then chances are she feels fine and is getting enough fluids. It stands to reason that a thirsty child would drink more water even if it wasn't her favorite beverage.