Okay, those are too too cute! I would say it would be pretty simple. A toddler sock pattern that continues up the leg. I don't think it would take much expiramentation for a knitter who is familiar with socks. But that's not me, unfortunately.
I made my DD (now 20 months) legwarmers for the winter for all those reasons they mention on the website. I did them ribbed the whole length, but you could just do the ribbing at the top and bottom and stockinette for the main part. I used KP Merino Style. I don't think the thin cotton that BabyLegs uses would provide much warmth, but they are certainly cute!