There's really not a lot of difference in head size between a newborn and a 9 mo old. Maybe a couple inches or so. So using a thinner yarn and smaller needles may work but check your sts per inch divided into the 72 sts to make sure that won't be too small. The hat uses a pretty small needle for aran and probably isn't any larger than 14" as written; using a worsted weight and size 7 would probably make it about 13" around which shouldn't be too large.
sue- knitting heretic