Once the elastic was secured with a second row of stitching along the other edge, I folded the wrong sides of the waistband together, marked the CB/CF and sides of it with pins, then matched those pins to the CB/CF and side seams (on little kids' pants, you don't have to be too picky about dividing the elastic into perfect quarters; it was plenty close enough). I sewed a row of stitching about 3/16" from the cut edges, then went around again zigzagging over the little bit of seam allowance.
She's wearing them now, under this outfit:
(Sorry I don't have a photo of her wearing them; she was "too busy with her projects" to let me take pictures, and frankly, they're not terribly exciting to look at under the dress.) In the above photo, she's wearing a cotton sweater dress I picked up for her at the Gymboree outlet, over a long sleeve knit shirt from Land's End (one of several items that arrived from Grandma yesterday). Just a few minutes ago, she came in to the sewing room to tell me that they were the most super-cozy pants EVAH (oh, and, BTW, could I help her find a Netflix Watch Instantly movie on the computer?).
*Yes, she was sucking up in order to get me to do something for her, but it was still nice to sew something that didn't get completely screwed up.