Ditto to all the books mentioned here. DS has read most of them around that age +/-. Esp. the Magic Tree house series. Another one I have to mention is these books by Louis Sachar:-

Sideway stories from Wayside School
Wayside School is Falling Down
Wayside School Gets a Little Stranger

They are hilarious! We had fun reading them.

When they are older and if they love Math, they will like Sideways Arithmetic From Wayside School too! smile