I Am Kind: A Little Book about Abraham Lincoln talks about the personality of Lincoln with the backdrop of being kind (a trait he was known for) as the moral to the story. And while being kind is a subject 
or moral for all ages to learn, the overall feeling of the book (the fact that it is being told by the point of view of Lincoln and dealing with in simple terms, the personality and events of Lincoln) seems a bit mature for the board book crowd. Nothing is inappropriate, but it is a book your child can grow with.
Perhaps Brad Meltzer’s book could be used in a classroom setting for ages 4 to 6 (perhaps even slightly younger) but if you focus on the kindness part and the other emotions presented and not so much who Lincoln was, this could go younger. However, Meltzer has multiple levels of stories and people, therefore there is something for everyone, one just must do a little research in finding them all. Yet, he covers from Jane Goodall to Walt Disney to Jim Henson to Rosa Parks.
Christopher Eliopoulos has some clever illustrations. They are sweet, light and cute. Not only does he illustrates the Ordinary People Change the World series by Meltzer, he has other books as well. This signature style can be seen on the pages of this and the other Meltzer books. Some images are serious and others not so much (imagine a child-like Abraham Lincoln pulling a “bugga bugga” face…).