Lucy & Andy Neanderthal: The Stone Cold Age (Lucy and Andy Neanderthal)
Andy in particular seems like a modern child transported back to prehistoric times, because he doesn't want to be crowded in the cave, he balks at skinning a deer, and he is generally unhappy with small things. Young readers will be able to sympathize with his plight.
The supplementary information at the end of the book gives additional information about the Stone Age, how we know information about this time period, and where readers might be able to see some of the artifacts.
There are a lot of modern sensibilities in the book, which make it very humorous, although I found it hard to imagine that the human father would have fainted when he saw a baby being born. The mothers are referred to as "Mrs.", and the characters all have very AngloSaxon names.