Ooops! I just skipped the pattern section of the question completely! Sorry!
I lived in southern Mississippi and Louisiana till abour 3 1/2 years ago. The patterns I used usually had some kind of open type stitch. You may want to make something with a stitch that creates a firmer fabric, but you can have fun with other stitches, too. I have a pattern book for sweater sets and layettes from Leisure Arts that I like a lot. I have also found patterns online or on the yarn bands that I like. I'll have to see if I can find links to patterns or at least pics of the patterns that I have used.
And congratulations on the new grand baby.
ETA: I made this pattern using Lion Brand Cotton Ease in the 6 month size and it turned out nice. Notice that it doesn't button all the down. This probably makes it more comfortable in warm temperatures.