I found this super cute pattern for kids t-shirt pants on rookiemoms.com
I like it b/c you don't even have to sew in a hem! It is already there!

Activity #361: Make some easy kids pants
The whole project was about 40 minutes. The full tutorial is here.

1. Identify an appropriate t-shirt or sweatshirt.

2. Fold a pair of baby’s pants with crotch sticking out and trace. I have outlined what it should look like for you. With this placement of the cut, you will not need to hem the bottom.

3. Cut on your traced line.
4. Cut a second pant leg from the other side of the shirt. Look for the arrow on the left in this picture to see how I used my first pant leg as a guide.

5. You have two pant legs now, right?

6. Place right sides facing each other and pin. Sew along curves.
7. Reorient the pants and you’ll see that you just have a crotch to sew up and a waist to fold over and sew. Leave a space to thread the elastic through.
8. Insert elastic, sew waist closed, and you are done!