Hat sizes for kids - Sooooo confused now

Hi. I am trying to knit a one size fit all hat size for 4 year old kids. Although, I see the general head circumference to be between 19 - 20. I also have read about negative knitting and I thought about doing a rib hat size 18 inches to give a closer fitting. Is that something you just play by ear? I found a pattern that matched to the super bulkly yarn I have, which is 2 stitches per inch and CO 30. I measured it and it turned out to be 16 inches in cirumference and fit perfectly snugged on MY HEAD which they said is for 4 year old. I’m soooo confused now. Should I knit the exact size and run the risk of the hat being too big or knit negative with a closer fit?
Any insight is appreciated!

You could use the hat you’ve knit as a large swatch. If the CO of 30sts makes the hat too large for a 19-20 inch size then you can figure out how many sts to cast on for the size you want. There are some hats here including a ribbed version with and without a cable that may give you some guidance too but the swatch with your yarn and tension is probably the best guide.
http://www.ravelry.com/patterns/search#photo=yes&craft=knitting&fit=child&view=captioned_thumbs&sort=best&weight=super-bulky&pc=beanie-toque
Remember too, that an adult woman’s head size averages about 21.5-22.5 inches, not that much bigger than a 4 year old. In addition, the great thing about an all-over rib is that it is very stretchy.

Thank you so much! Actually, the CO 30sts fits my adult head perfectly in rib trim and stockinette body. I’m just thinking maybe to add a couple of more stitches for some give. I will review the link you sent me. Thank you.