I am always delighted to help find books for kids. A young USAF family being reassigned from Japan sent me an email asking for book recommendations for Japanese themed and Japanese/English, bilingual books. These are the books I’d recommend to keep the kids exposed to the Japanese language.
I just found out about Tuttle Publishing and they sent me the perfect book!
My First Book of Japanese Words: An ABC Rhyming Book by Michelle Haney Brown
This is a lovely picture book in ENGLISH but with Japanese words for each letter both in Hiragane and Kanji. There is also interesting tidbits of information about the Japanese culture that kids would enjoy. [picture book, ages 2 and up]
Japanese Themed Picture Books for Preschoolers
Here are more picture books with a reference to Japan.
Japanese/English Bi-Lingual Folk Tale Picture Books
I happen to have these Japanese Folk Tale books that I found in a Japanese bookstore in Orange County when visiting my mom. They are lovely little books on famous Japanese folk tales and are bi-lingual.
These books are great and have Japanese vocabulary words throughout both books as well as cover the holidays and traditions of the Japanese people. I learned a lot from them!
The Story of Yuriwaka: A Japanese Odyssey is a short chapter book which would be good for the older son.