Creating a model of the solar system is both fun and educational for young children through teenagers.
This is an activity I've used in conjunction with science lessons and as a reward activity.
All you need is a set of Styrofoam balls, paint, pictures of each planet and some fishing monofilament line for hanging the balls when they are completed and dry.
Most craft stores carry a set of specifically for solar system models and small bottles of various colors of craft paints.
Before you buy the paints you should do some research unless you want to buy a wide variety of colors to be used for other projects in the future.
The internet is the best source for photos from NASA showing what the surfaces of each planet look like.
For older children able to search the internet, they could find out what is known about each planet.
The information the child learns could be turned into a "solar system booklet" suitable for science fairs or sharing with others at school.
Essentially, this is an opportunity for improving writing skills.
Many states require students to have portfolios of the various types of writing.
A report is an example of expository (non-fiction) writing that could be included in the child's portfolio.
Adults can help their child with this type of activity by making sure there are grammatically complete and correctly spelled sentences arranged in paragraphs (including topic sentences and supporting details for each paragraph) and an introduction and conclusion for each planet's page.
The painting process can turn into art lessons surrounding the mixing of paint to achieve the correct shades, hues and colors of the planetary surfaces.
I hope you have better luck than I do - I'm an expert of making shades and hues of mud, but many planets have those colors I mistakenly mix.
No matter where you choose to do this project, make sure you have lots of newspaper to absorb spills (and layers can be peeled off should a spill occur so that what is being worked on doesn't get adversely affected) and water for clean up available at a moment's notice.
Fingers will get painted because the child is moving the ball around to cover its entire surface.
It might be helpful to place the ball on an empty plastic container (yogurt cups are great for most of these balls) as they paint.
That way the ball only needs to be handled once when it is rolled over top to bottom.
Let the balls dry completely before starting the hanging process (ceilings in the child's room may be where these reside).
While they are drying, you can figure out relative distances between the planets.
Start with deciding where the "sun" will be before placing Mercury as close as possible.
You'll then have to work out ratios of distance that will depend on the space you have available.
The ratios don't have to be exact, but try to get them hanging in relative distances from each other and the sun.
When I had this in my classroom, I didn't have the space to show the distance between the sun and Pluto (it was a planet then), so it was just as far away as we could place it.
The students knew it was far out there and that was what mattered.
Have fun with the project!