Re:Creative Birthday Desserts for a Dad on Budget
It's a cake, but you could try the Rainbow cake from YouTube. I made it for my own birthday a couple of years ago and it was beautiful. You need a box of white cake mix, a can of white frosting (or make from scratch with a box of confectioners sugar, stick of butter, a little milk and vanilla extract), sprinkles if desired, a four color box of food coloring, and eggs and oil for the cake mix (I like Duncan Hines brand the best.) Unless you have to buy the eggs, butter, AND oil this should easily be under your $12 limit.
To make the cake, divide the prepared cake batter into six small bowls, and color them the colors of the Rainbow using the food coloring: red, orange, yellow, green, blue, and purple. Make them nice and bright. Then pour the colors into your greased and floured cake pan one at a time, starting with the red, in the order above. Pour each succeeding color on on top of the previous one, right in the center, so that it makes a circle with the previous color showing around the edge. You should end up with all the colors showing in concentric circles. Place carefully in the oven and bake in the usual way. Frost and decorate with sprinkles if you like. When you cut the cake, you should see a beautiful, arched rainbow! This might be a fun project for your son to help make, too.