It depends what her interests are. Does she have hobbies, like music/art/blogging/nature? Or you could give her money or jewelery. You could also take her around town and get her what she would like to choose. A nice dinner is something she might appreciate too.
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
legos or transformers or a toy truck
A really cute Scarf and hat.
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.
digital camera cell phone laptop 50 dollars