Chirri & Chirra Under the Sea by Kaya Doi
We adore these books. Kaya Doi has made the most sweetest series of picture books (translated from Japanese by David Boyd) about two sisters and their adventures.
The two often set out on their bikes (Dring Dring! Dring-Dring! go their bike bells!) and meet up with all kinds of creatures, animals mostly, living under ground, in an icy land, a grassy meadow, or in the newest book under the sea. There's always some sort of magical element to the drawings - something that children can easily imagine and bring to life inside their heads. A favorite moment of mine is when the girls play marbles made of frozen ice with flowers inside, only to melt the marbles in their hot bath afterwards when the ice cubes melt and release the little flowers... its moments like these that make the books a delight to read over and over again.
Each books stands on it's own, but you'll likely want to collect the whole set!
Best for ages 3-6yrs.