This is a perfect introductory book that highlights the celebrations & festivities during the 5 days of Diwali.
Especially perfect for toddlers, as it's a board book, and they'll love the colorful illustrations!
Though it's a great read for older kids as well, Indians & non Indians alike. In-fact this would be the perfect book to take & read to your kids class (especially if you are living outside India) to introduce them to the festival.
Also, this particular book doesn't get into religion, just the festivities & celebrations on and around Diwali.
For the religious reasoning for the festival, you can check out - Amma Tell Me About Diwali.