Reddit reviews A Place for Pluto

Oh my gosh, you totally have to get him this book!! I just saw it at Barnes and Nobles the other day.
A Place for Pluto

I'd recently made a similar post elsewhere, so I'll go ahead and copy that below. In addition to that, another thing my four year old and I have done is to make a scale-distance solar system on the sidewalk using chalk. Look up the distances of planets in AU, draw the Sun, and then get walking--one large step = one AU. Don't worry about properly scaling the planet sizes themselves because then everything would be tiny. Feel free to include fun details like the asteroid belt / Ceres, large moons, dwarf planets, etc. And at the very end, Voyager. Its amazing to see how far out that one has gone!

So many good books to choose from! Examples:

• Moon's First Friends
  
  
• The Sun is Kind of a Big Deal
  
  
• Hello World - Moon Landing
  
  
• First Big Book of Space
  
  
• There's No Place Like Space
  
  
• A Place for Pluto
  
  Other book recommendations here: http://www.planetary.org/blogs/emily-lakdawalla/2018/space-books-kids.html
  
  Looking through a telescope is another fun activity you can do. But please don't bother with a "kids telescope" - those are basically toys, are difficult to use, and barely help in seeing even the Moon. The smallest real telescope is around a 4.5" - 5" reflector. This will obviously be too large for a 4 year old to operate, but with your help he will be able to see things first-hand. If you are unfamiliar with the sky, free apps like Stellarium or Sky Safari (iOS) can guide you where to look. Jupiter and Saturn are currently out in the night sky.
  
  View the International Space Station! See here for when the ISS will be making a pass over your area, and explain what he is seeing!
  
  Follow current space events! This can include watching rocket launches (SpaceX does a great job with webcasting these) or NASA/ESA science missions (Mars 2020 launches next year, and those landings are always very exciting).

With that said, [this is a solid children's book](A Place for Pluto https://smile.amazon.com/dp/1684460042/ref=cm_sw_r_cp_apa_i_VTxACb9MEGP10) that does a good job of explaining differences between planets, dwarf planets, moons, and more.