The neck of the size 6 fit easily over my head, maybe in part because I used the Old Norwegian cast-on which is very stretchy. It also gaped at my neck. But when I picked up the garter stitch neck to finish, it was way to small for a 5 year old child.
I re-knit, picking up the same number of stitches called for in the first increase round instead of a one-for-one stitch pick up.
I then knit two rounds, then purled two rounds. Knit one more round, then did the Icelandic bind off. I was amused that this made a little collar that insisted on folding down nice and neatly. It still snugly fits over my head, so I know it will fit over Logan’s noggin.