Looking for a book about ethics for children suitable for a secular family? How To Behave and Why by Munro Leaf will fit the bill. Written by the author of the classic children's picture book The Story of Ferdinand, it was first published in 1946, but is still as relevant as ever today.
The illustrations are simple stick figure drawings in bold black, white and red. The text uses straightforward language to get its message across, which is, briefly, that there are four things a person should be to have a happy life: honest, strong, fair and wise. It goes on to explain what these things mean and how behaving well will make you and the people around you happier, which makes a nice change from trying to motivate children out of fear or guilt.
While pitched at young children there is plenty here for older kids and adults to learn from too, such as the fact that no matter who you are you can't always be right.
I recommend this one for kids aged four and up. If you enjoy it there are others by the same author, including Manners Can Be Fun, Reading Can Be Fun, Brushing Your Teeth Can Be Fun and How To Speak Politely And Why.