This book hit the mark with our 7 year old and his many many questions on the Solar System.
Detail level is exactly right for age-group, and it is up to date with info on Pluto
The lift-up Flaps are a great age-appropriate learning device, too.
Bought this with a number of other similar books, and this is the one that keeps being delved into, including by a 5 year old sister who was not previously interested in what is out there beyond the Earth's atmosphere
Definitely one of the better books out there for this age range on the Solar System and would make a really great gift, too