Here's what I'd suggest...
1. Carefully measure your daughter. Realize that you're going to have to add some inches for "ease" (which will give her moving room in the coat) and also add a bit more for growing room. She is really NOT going to want to grow out of this thing in a few weeks. It's a beautiful pattern and she'll probably save it for her own kids one day!
2. Do a very thorough gauge swatch to nail down exactly the gauge you need for the project. For wearable objects, especially one as important as this one, leave nothing to chance. Swatch! Get your measurements for both stitches to the inch and rows to the inch.
3. Sit down with some knitters graph paper (the squares are a bit wider than they are high to mimic the actual size of the stitches) and color in the boxes, using your gauge from your swatch until you have the measurements totally lined up and visible on the graph paper.
4. Then translate the number of rows and stitches to work with your pattern. Write it all out before hand so that you can cruise right through it.
I know this seems like the long way around the barn, but you've got such a good chance of producing a fabulous piece that could well become an heirloom, and all at the beginning of your knitting career!
Best of luck and please keep us posted on your progress.