For the sweater, I started with this pattern from Lion yarn as a general size guideline. I used a size 8 (US) circular needle for most of the sweater, switching to a size 6 for the neck. Rather than the garter stitch edge on the bottom and cuffs, I went with a 2x2 rib on the bottom an a 1x1 rib for the cuffs. I also increased the button/buttonhole band to 4 stitches. Since I was making a matching hat, I chose not to do the hood and instead finished up with a 1x1 ribbed neckline.
For the hat, I started with a 3 stitch I-cord for a few inches and then quickly increased to a generous baby sized head circumference. Then I switched to a 2x2 rib that I continued to the end of the hat, making it long enough to fold up at the bottom. I folded the I-cord into a loop at the top and secured it with a ribbon bow. I think for a boy I would have knotted the I-cord and left the end sticking up instead.