Child Helmet

I've got two kids (girls), one 10 and one 8, they are both quite big for there age I've seen some helmets on offer at intobikes my question is there any difference between childrens and adult helmets other than the size?

The kids are currently coming up as:
8 year old - Small
10 year old - X-Small

Funnily enough my misses is XX-Small (reckon it was just cos she wanted a Shoe helmet )

Any advise would be much appreciated.

Cheers guys (and girls)