At their age, they're about to plunge into there teenage years, so lip gloss and basic makeup will fascinate them, also they want to be treated like an adult, so no sparkly or little girl makeup.
Another thing is a cellphone, a minifridge, or room accessories.
A ring or flowers. Mabye a gift card to a store.
an iPod
A 6 year old would most likely like a stuffed animal.
Gift Cards
You can buy fashionable and lovely clothing as a Christmas gift for a 20 year old girl at the Durant Square Mall in Oakland, CA.shopping online ,I think the brands good : oasap.com,forvere 21,H&M and so on,cheap and good clothes
If he's your friend then he any gift would be a great gift so yes^^
Buy her clothes or jewelry. I am 11 but i still love that. If she is more tomboy get her video-games and stuff.
That really depends on what their interests are.
A case of beer.
Anything
A really cute Scarf and hat.
legos or transformers or a toy truck