I highly recommend getting a class 10 SD card if you can afford it. i've heard of a toooon of kids having problems with class 6 cards; the camera just stops recording because the card can't keep up. class 6 will get the job done most of the time, but class 10 will run more smoothly and make your life so much easier.
i love my SanDisk Extreme SDHC Class 10 16 gb cards, but i also recently bought Kingston ultimate X 120X SDHC Class 10 16 gb cards that have been solid as well