The arms of the mobile are cut from wire coat hangers, with a loop bent in them off-center. The strings are just some heavy black thread I had kicking around. The twirly bits are images I had on hand, printed on heavy inkjet photo paper and mated up back-to-back with double-sided tape (the tape also holds the string). A little black magic marker around the edges hides the fact that I can't cut a perfect circle even with a pair of good scissors and the mad inspiration that can only come from paternal love.
On one arm we have the Earth and the Moon. On the other arm is Mars and the Jovian moon Europa. I guestimated the length of the arms, then attached the cutouts and fiddled with their positions along the arms until things balanced. I wound up trimming a bit of wire off of both secondary arms to compensate for the weight disparity between the planets and the moons.
Total project time was about an hour. Asha is very pleased, and she tells me that the little guy has spent a fair amount of time looking at it. Yay!