I have a 5 year old girl and 2 1/2 year old boy. Some of our favorites are:
Sandra Boynton - Hippos Go Berserk; Green Hat, Red Hat; Oh My Oh My Oh Dinosaurs; Moo Baa La La La
Laura Numeroff - If You Give a Mouse a Cookie, If You Give a Pig a Party, If you Take a Mouse to School, etc.
Byron Barton - Trucks; Machines At Work; Trains etc.
Kevin Henkes - Owen; Kitten's First Full Moon
Bill Martin - Chicka Chicka Boom Boom (letters)
Mick Inkpen - Kipper; Kipper and Roly; Kipper's ABCs
Dr. Seuss - Go Dogs, Go!; The Foot Book; Green Eggs and Ham
Girly ones - Fancy Nancy (there's a few)...Pinkalicious and Purplicious... Angelina Ballerina (series)...Cinderella...
Boy favorites - Digger Man; Harry the Dirty Dog; Potato Joe
Bedtime/naptime - Goodnight Gorilla; Time for Bed; Going to Bed Book; Snowmen at Night; How Do Dinosaurs Say Goodnight? The Napping House
They might like books with textures to touch or "lift-the-flap" books where you find pictures underneath. They also might like ones where you have to find something little on every page..like a little duck or ant.
Have fun and happy reading!!