For 3-4yr olds:
Arnold Lobel - you can try the Frog and Toad series, Mouse Tales and Uncle Elephant. All are excellent. They are also good beginner reader books to pick up if your little one is on his way to reading independently.
Shelley Moore Thomas's Good Night Good Knight, and Get Well Good Knight - they are hilarious and the kids would certainly love them.
Harper Collins Treasury of Picture Book Classics - this hardcover book is excellent and contains all the classic tales like Goodnight Moon, Pete's a Pizza, Caps for Sale, Leo the Late Bloomer, Head to Toe, Frances and her Baby Sister, and many more. It was through this treasury that I discovered many wonderful authors that have become the girls' staple.
Shamini Flint - her Sasha series - we have both sets - Sasha in Singapore (which covers the Bird Park, Zoo, the museum, and the Botanic Gardens) as well as Sasha in Asia (Hong Kong, Kuala Lumpur, Bali and Bangkok) are great in introducing young children to sights and sounds around them.
Other all time favourites include Eric Carle's A House for Hermit Crab, Sendak's Where the Wild Things Are, and Martin Waddell's Owl Babies
For 5-8 yr olds
We had recently picked up the Magic School Bus series, a great beginner science series. I saw them quite a while ago, but haven't been convinced, although many of my friends had been going on and on about it. It's begun to grow on me though. I find the pages very busy, and for me, I'm not sure where to start looking. But kids somehow find them very appealing. Each story explores a particular theme - the solar system, the ocean, inside a beehive, under the earth etc. Very informative and presented in a fun and colourful manner.
I started exposing R at 6yrs to different genres and more non-fiction. I picked up Mary Pope Osborne's Magic Tree House series, and we were hooked. R to date has completed at least 10 books exploring themes like Titanic, Pompeii, the American Civil War, how the Olympics came about, the Dinasour age, Mummies in Egypt and much more. Fascinating for older kids. What's great is each story is spun around the adventures of Jack and Annie, siblings who discovered a magic tree house that could transport them to different eras. Kids thus uncover historical facts in an interesting way. The great thing about this series too is that each title is accompanied by a research guide which allows the kids to delve deeper into the facts if they wish.
Roald Dahl - ahh.. who can resist him ? I started reading to R Charlie and the Chocolate Factory when she was 5.5yrs and she was hooked. Since then she has read The Enormous Crocodile, George's Marvelous Medicine, James and the Giant Peach, re-read Charlie on her own, and has just started on BFG.
Hope these give you a headstart as you head out to the bookstores or libraries. Have fun browsing !