Aren't these adorable? I wish I was 4! I used the undies pattern from The Red Kitchen. (The link is here but it looks like you can't buy this pattern anymore.) I made the 4T/5T size and used some cute t-shirt scraps. This is definitely what I'm doing with all of the kid t-shirts that end up wrecked from now on!
This was a nice easy pattern. The instructions have you zigzagging the top elastic casing but I only did that on the front of the panda pair. For everything else I used the existing hems of the shirts and just cut my elastic in half so I could thread it before sewing the side seams. You can see below how much nicer it looks. (I love cheating that way, go t-shirt refashions!) I may try adding the white trim to the waistband as well next time.
The only problem I had was that I brilliantly stuck my finger too far in and sewed myself half-way through. I made my husband run around the house after the tweezers and then hopped around in front of him refusing to let him pull the needle out. (I did it myself. Travis was highly amused. I push out babies without drugs but if I get so much as a splinter I am an enormous wuss. Go figure.)
Oh, I almost forgot! My favorite part of this pattern is that they have you fold over a little piece of the white t-shirt and sew it in the back as a tag. (We grown-ups can tell from the shape of a garment which way is front but that's a lot to expect from the littles.)
Possibly not the most glamorous thing to sew but I'm finding that Sophie's store bought ones are already disintegrating and she hasn't even grown out of them yet. I'll have to whip up a couple more pairs for her. What, what? Who are they for then? They're part of Katie's birthday present. Four because she will be four :) I have a couple of other fun little things to tuck in with them, some dragon stickers and these paper dolls, but undies are still amusing at 4.
Shh... don't tell, she hasn't seen them yet ;)