My daughter is four, but still loves the same things as when she was 3. Anything princess, anything to do with crafts/coloring/painting/playdoh, dolls, dressing up, and playing pretend.
Here are a few ideas for the 3 yr old for you:
Play doctor's kit- they have a FP one at Toys 'R Us - she loves playing doctor with her dollies
Doll clothes/bottles/accessories- they have little diaper bag kits with the "necessities" or a doll bed or doll stroller
Games- Chutes and Ladders, Candyland, The Ladybug Game, etc.
Glitter Paint/Princess/Tinkerbell or something similar
Anything for dressing up or playing pretend- fairy stuff- wands, skirts, princess stuff, kitchen stuff, a kids apron, toy food
For the 10 year old, it depends on what she's into-
Anything crafty that she can make (they have kits at Walmart, Michaels, and Target, too, I think)- maybe bead jewelry that she can personalize
Books- Nancy Drew, Choose Your Own Adventure
Games- not sure which ones, but there are a ton out there
Good luck! I'm sure whatever you give will be great!