I ue bias tape just to really reinforce the armholes. Those kids have maybe one outfit, and you figure that it may get washed in a rough spot like a river or a bucket! I have seen pictures of those kids dressed in rags, and I am sure once the dress wears out, this is what they have to wear. I do whatever I can to make the clothes last. I also do not use buttons or zippers because they may not have a way to repair them.
The bias tape can get expensive, but I recently bought a little inexpensive device that you can use along with an iron, to make your own tape. I plan to cut some sheets into strips, and use those to make the tape. it will only cost pennies. Maybe you can try that too!
There are lots of free pillowcase dress patterns. I am with the other ladies. try Nancy's Notions. Her pattern works really well!