hat question

[knitaholic]

I need to knit a hat for an 18 mo. old child--I have a favorite pattern, it's knit flat, but I would like to knit this in the round. My pattern states to cast on 68 sts--how many would I cast on for circular knitting? I do not know the circumference of the the child's head, but the pattern I have is Bev's Really Stretchy Hat (it's k2p2 throughout). I know I wouldn't need to cast on as many stitches