I love when King Cole patterns span a big size range. This is from around 3 months to about 7 years old, and it's an easy knit pattern, so this is something you could work up pretty easily and quickly throughout all stages of this kid's life. They won't grow out of it before you've finished it! You get the option of a hood or just a round neck collar, the hat fits an "average baby head", and this is endlessly customizable in all kinds of solid colours or crazy self stripers.
For solid shades, I'm going to recommend either King Cole Pricewise DK for the durability and washability you get with a 100% acrylic yarn. But, you could use the King Cole Cherish DK for a self patterning soft yarn, the King Cole Fjord DK for an even more striking faux-isle patterner, or the King Cole Bramble DK for a proper good stripe.