I found a pair of unwearable cropped pants at the Bargain Barn the other day and decided the thick material would be perfect for my active girl.
Start by cutting the back and front completely apart along the side seams and inseam.
Fold the pants front in half and, using one of your child's pairs of pants as a pattern, lay it out making sure to match up the top and crotch seam (1 and 2). Cut around the pants, leaving the crotch seam untouched and adding about 1/2" to allow for seam allowance (3 and 4). Also cut the pants longer in order to accommodate a hem.
Cut off excess material (i.e. front pocket linings - 5) and remove button loops unless you are satisfied with where they are positioned (6).
I chose to remove the back pockets on these pants (1) since they seemed too big once I cut down the back. Fold the pants back in half and position the pants you are using as a pattern over them. If you are keeping existing pockets, you'll want to center the pants over them (2). You will need to have the tops of the pants even before you cut, but you will probably need to cut the back seam. We will be adding elastic to the back of the pants to insure a better fit, so if the pants you are using as a pattern don't have elastic in them, add a few inches to what will be the side seam. Again, cut around the pants leaving about 1/2" extra for a seam allowance (3 and 4).
Sew seams together:
Reassemble the pants, starting with the back seam. Pin the two back pieces, right sides together (1), making sure to carefully match any topstitching (3). Stitch together (2).
Next, sew the side seams together. Again, with right sides together, pin the side backs to the side fronts and stitch.
Finally, sew the inseam, again pinning the inseam of the front to the inseam of the back and stitching together.
Cut a strip of material (using either scraps from what you've cut off of the pants, or a contrasting fabric) that is the length of the waistband at the back of the pants and the width of the waistband plus about 1" (1). Turn under the long sides of the strip a 1/2" on each side and iron. Pin to the inside of the waistband (3). Stitch to the waistband by following existing topstitching (4). Only stitch the long sides, leaving the short ends open. Insert elastic into the casing you have created and try them on your child to find the amount that will make them best fit. Sew down the ends of the elastic along with the short ends of the casing.
Turn under about 1/2" of the bottom of the pants and press (1). Turn down another 1" to 2", depending on how generous of a hem you want, and press (2). Topstitch the hem (3).
The existing fly opening will probably look too long for your newly resized pants. Making sure the fly is completely zipped up, shorten it by topstitching a little farther up, following the curve of the original topstitching (2). Pick out the topstitching below the new curve you've created (3). The marks from the old topstitching will be unnoticeable after a few washings.
If you wish, sew belt loops back onto the waistband (4).
If you removed the existing pockets and wish to add new ones, you can do so at this point.