Okay... here's what I'd do...
I'd make a round cake & frost it yellow. Then I'd put it on a big platter (much bigger than the cake itself). This is the SUN... pipe a bunch of frosting "rays" coming off the cake & onto the platter. (Or instead, you could frost pointy ice cream cones yellow & stick them to the sides of the cake.)
Then I'd make a batch of cupacakes (but you'd only use 7 or 8... depending on
if you wanted to include Pluto!)... I'd stick a skewer in each cupcake & attach a "planet" to the top of the skewer. You could buy & paint styrofoam balls (or have your SON DO IT)... or you could simply find jpg's of each planet, print them out & attach them to the skewer.
Then just put the cupcake planets around the big sun... and VOILA.
(you could also decorate each cupcake to look like each planet... but I think it'd be more eye-catching/cool with the planets "floating" on the skewers.)