For Sophie, whose favorite things are space, making up stories with her playsets, solving mysteries, and making things:
5t pajamas
Scooter
Roller skates
Lunch box
Lego Friends Camper
My Little Pony bedspread (twin)
Basic builder set of Legos or just about any sets of Legos
Space-related stuff, like spaceships, or astronomy things.
Art Supplies
Shrinky Dinks
Sophie is learning to read and write and her numbers, so learning toys based around that are a good idea (as far as counting, she's working on numbers over 10)
For Owen, whose favorite things are animals, big machines, books, and pissing off Sophie:
Adjustable bike helmet
2t pajamas (3t if summer weight)
toddler bedclothes
Elmo stuff (might be on clearance!)
Cool socks
Thomas stuff
Kid-sized push broom
Baby doll and a crib for it to sleep in
Playmobil 1-2-3 sets (they already have the fire station and truck, speed boat, zoo, rocket, and a couple of Christmas sets)
For either:
The Imaginext stuff that they have gets played with a lot by both kids, so I expect more would go over well.
For both:
Dress up clothes (firefighter, cowgirl, etc.)
Bathroom stuff to match their shower curtain
Good toddler markers/crayons (Crayola seem to be the best)