Oh my gosh, I'm getting such happy flashbacks reading all these book names! The older ones I read myself as a kid, and the newer ones I read aloud to my children.
Here are a few of our favorites - some may have been mentioned already.
The Indian in the Cupboard series (they got better as they went on! The Mystery of the Cupboard, which I think was 4th, was absolutely fantastic.)
Ella Enchanted (Best re-telling of the Cinderella story EVER. Ignore the stupid movie!)
Where the Red Fern Grows (saddest book OF ALL TIME)
Two Princesses of Bamarre
My Father's Dragon and sequels - first full-length books my daughter ever read herself.
Redwall ... not all of them as it goes on forever, but the first few were fabulous.
Holes (the movie was great, too!)
Ender's Game - I know it's not really a kid's book, but I haven't yet met a 12 or 13 year old that didn't adore it.
More recently, my kids have loved
When You Reach Me by Rebecca Stead - lovely time-travel tale! Reminded me a bit of "Wrinkle in Time" in its writing style.
Tunnels (and sequels)
And one of the coolest children's SF novels I've ever read -
Solay by Mark Jonathan Harris ... I don't think I've ever met anyone else who's even heard of it, but we all loved it!
Oh, and for classics that I read as a child and then read to my kids:
Follow My Leader
The Enormous Egg
The Shy Stegosaurus of Cricket Creek
The Lost Race of Mars by Robert Silverberg - the very first SF book I read myself.
ETA: And how could I forget:
Mr. Bear Squash-You-All-Flat!

A favorite of all my kids.
ETA again - how about
Half Magic or others by Edward Eager, and
Five Children and It or any by Edith Nesbit?