Fit issues

I have finished knitting the body of the Knoroyven, but am having trouble with the neckline: I have no idea what size the average 2 year old skull is, but I think it's significantly larger than the neck opening was once I cast off. I have ripped back and re-cast off this neckline about 4 times. I tried a larger needle (too small), a picot bind off (too floppy), a "stretchy cast off"(too small), and the "stretchy cast off" with a larger needle (still too small). Somerset couldn't bear to watch it anymore. I plan to try an i-cord bind off next. I also demonstrated that gauge can affect baby garments: This is a BSJ knit at a gauge of 7 stitches per inch - newborn size is supposed to be knit at 6 stitches per inch. (In my defence, this was a travel knitting project - I guessed at the needle size that would work, and did not bring a ruler along. So it wasn't until I got home that I could measure my gauge.) Again, I am quite sketchy on how large the average baby is, but I am thinking that the jacket should be bigger than a skein of yarn (not that it should use a skein of yarn, but the dimensions should be greater). I guess this size would work OK for a preemie, but given that the baby is due some time in June, that doesn't help me.