I just re-looked at the pattern - the part that is curling (the "bottom" edge) will be sewn together. The curl will try to pull itself to the outside, but will have nowhere to go. The other end flows straight into the yoke, so will have nowhere to curl either.
I debated making this in a different yarn. I know acrylic is notorious for not blocking, and stockinette curls. But machine washable.... I think a hand knit should be used, and that baby clothes should be able to be easily washed, otherwise what's the point of having it? I would hate to have put a lot of effort in for a friend and have it go to waste, you know?
Whoever designed this was a smart cookie. The hood edges, which are currently the sides of the flat piece are NOT curling due to the 4 garter stitches on either end of the rows. Although, as I do tend to drone out knitting, I had to put a marker before the last 4 stitches in the purl row - before I did so I found myself purling to the end of the row and tinking at least three times.
I think it'll be good, and good practice. And I'm going to quit threadjacking now....