This activity is like sliding ground spinach into your kids' hamburgers when they aren't looking. It's good for them, but they unknowingly enjoy it. Just remember, without Pluto, it's no longer "My Very Excellent Mother Just Served Us Nine Pizzas." It's "My Very Evil Mother Just Served Us Nothing."
Others have gone before you, so no need to reinvent the wheel. Here are five different projects to take your pick from:
1. This solar system is part sculpture, part educational model, and all amazing-ness from Pinta La Lluna.
2. This miniature solar system, courtesy of Instructables, actually revolves.
3. A solar system made out of buttons! It's deceptively educational.
4. It's a very Montessori solar system.
5. Styrofoam balls make up this 3D model, via Crafts for Kids.
Have fun and happy building!
(Images: as linked above)