Girl:
Jewelry
lotion
perfume
make up
lip gloss
shoes & clothes
Guys:
skateboard
headphones
Sport gear (if he's in any sport)
iTunes card
DC hats and/ or shoes
Gift Cards
That really depends on what their interests are.
A case of beer.
A ring or flowers. Mabye a gift card to a store.
an iPod
Toys,iPod,basically anything electronic
A really cute Scarf and hat.
legos or transformers or a toy truck
If the 11 year old is responsible, it is a good gift. They are a little pricey, so I would base it on how he has treated his other belongings previous to this.
A good Christmas present for your 15 year old girlfriend can include perfume, scented body lotions, and affordable jewelry. Another great gift would be a gift card to her favorite store.
A 6 year old would most likely like a stuffed animal.
iPod shuffle. They are often available for around $40.