I’ve made a hat from this pattern (That Easy Guernsey Hat by La boutique de Jeanne) before, for one of my great-nephews. It turns out a little bit big for a newborn, but could be worn, and should also work for a 6-month old (say, if the child is born in the summer months).
I used 100% acrylic worsted weight Knit Picks Brava in the color Denim.