I love working with t-shirt fabric for my kids costumes, it's easily available and comfy when worn. Here's a quick tip for adding elastic to the collar, sleeves and the bottom.
You will see that there's already a hem, all you have to do is make a little slit in the inside (make sure it's only in the inside and that you're not cutting the right/front side) and add your elastic and sew it up.
This was really helpful when making a dozen reindeers this past Christmas!
