Measure the child and choose a size based on her chest width and height. For a perfect fit I recommend that you choose a size which has a finished garment chest width that is 2 cm less than the child’s current chest width.
It’s perfectly all right to mix sizes and for example sew size 116 in the bodice, but size 128 in the skirt/pants, just don’t forget to also trace the waist/width from size 116 when tracing the pants/skirt (see info in Sewing Instructions).
Victoria is designed for fabrics with at least 30% stretch. There is a stretch guide in the pattern that you can use to evaluate if your fabric is suitable.