My 5 year old son wanted this book and we got it for him. He is really into prehistoric animals and it is hard to find books at his level, so we decided on this photo journal thinking that the pictures would be nice for him.
1) The pictures can be a little gory. I have seen worse, so this book is fine for 5 year old level, but definitely something to consider.
2) The story in the book is actually for older kids (talks about lots of killing and hunting) so when we read it we "G-rate" the story, which is a lot of work.
3) Generally, it is hard to find factual prehistoric animal information for a younger (4-6) level, so we do the best with what we've got.