For under $10, they could buy a Disney beanie - my DS bought a Tomorrowland Mickey, and I'm pretty sure it was $7 or $8. I also found Space Mountain t-shirts for both DSs in the gift shop reduced to $7.99 - snatched them up in a hurry!
Most of the pins are under $10, and there's enough of a variety to keep anyone happy.
Also bought for under $10 (not sure exactly how much, though):
Kermit the frog (also other characters, obviously...) crazy straw
Soda mug (not refillable) from MK
Magnets
Key chains
Magnetic bumper sticker (OK, that was for Mom!)
If you can, try to resist the urge to buy the spinning lights sold at parades and Fantasmic - I think my DSs spent $12 on Winnie-the-Pooh spinning lights, and they only seemed to work for that week (actually, I think they didn't make it through the week). Of course, YMMV...
I was able to buy a package of 24 character crayons for the boys to bring back to their class for $10 - that went over pretty well! Also a big hit was having the character(s) sign a post card and mailing it to the class.
Hope this small list helps. Enjoy your trip.
Sis-bam-BOOM!
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Gail a/k/a CookieGVB
"All you need is mugs..."