Just have your kid try on her shirt, figure out where you want the bottom of the shirt to reach, and then measure about two finger-widths below that point and mark it. Lay the shirt out neatly on a cutting mat, and cut the entire bottom of the shirt off at that mark.
The cut edge will roll a little over time, which is normal for jersey cotton. This is why you cut it a little long in the first place.
Fix too-long socks. You could hand-sew this fix, and also use the cut-off portions of the socks as arm cuffs.
Have your kid try on one sock. The top of the sock should sit at the divot just below the kiddo’s knee, so mark a point a half-inch above that spot.
Lay out both socks next to each other on a cutting mat, and cut off the tops of both socks at that mark.
Turn the cut edges a half-inch to the inside of each sock, and stitch down with a zig-zag stitch.
Measure your kid around the waist, and divide that number in half. That number, plus a half-inch seam allowance, is the width that you want for your shorts.
Lay the shorts out on a cutting mat, and figure out by how much they’re too wide. Divide that number in half, and that’s the amount that you’ll trim from each side of the shorts.
Cut parallel to the side seam, all the way down to the bottom hem of each leg, to trim that amount from the pair of shorts on each side.
Turn the shorts inside out, pin the side seams right sides together, and sew. I stitch these twice to keep them secure–the last thing my kid needs is to bust through her shorts diving for a pop fly!
My daughter is MUCH more comfortable in her uniform, now that it fits properly. I also attribute her base hit and her three successful throws to first base in tonight’s game to these uniform alterations, just so you know.