I'm a 14 year old girl, and have a 10 year old brother, so lets see if I can help you out.
Boy- if he's active, like my brother, he might want a basketball or some tennis balls he can play with. Candy is also very good, perhaps throw that in. Put some pool toys and arcade cards in there too, if he's anything like my brother. He might also want those washable tatoos- my little bro had a thing for them a year ago.
Girl- she's almost a teenager, so obviously she's getting into fashion and the whole teenage scene. Get her some jewelry and CDs, if you can't find any good CDs, Disney Soundtracks are always fun to listen to. She might also want some makeup, so maybe get her some lip gloss. That's about as far as I'll go with a preteen- no mascara or eyeliner until she's a teen. Also get her a pair of flip flops- comfy ones, and not with wide straps- because girls live in these. A magazine would be good too, maybe GL or ElleGirl or YM- magazines that are appropriate for preteens (Cosmo Girl, Teen People, and Teen Vogue are a little too old for her, and American Girl is too young). A pair of sunglasses would be good too. When in doubt, get her more jewelry.
Hope this helps! Of course, you can't make a perfect gift basket unless you know what they're like and what their interests are. For one thing, the boy could be totally uninterested in sports, and the girl uninterested in fashion.
EDIT- I have no idea what the pricing is for all this stuff, these are just suggestions to go on for the ages of the kids. Truthfully, I have never done a gift basket but just remember what I wanted at that age and what my brother wanted.
