Go to the store and pick out different sized styrofoam balls. Then, stick them onto cocktail sticks. After that, your children can work together to paint the planets in correspondence with the solar system.
Alternatively, if you would rather stick your planet balls to poster board instead of in skewers, cut each styrofoam ball in half and then it’ll stick nicely to your poster board. Just make sure you paint your balls before sticking them to the board.