There is a really easy way to make the cakes without costing a fortune. It's very simple. The mixes are basically 2 tablespoons of cake mix, any cake mix. For birthdays and stocking stuffers, I packaged up a bunch of the mixes in plastic ramekins (available a Gordon's Food Service). Then, I slipped them into a tulle bag. To make the tulle bags, you just use the tulle by the spool (it's like a $1 for 100 yards or something like that). You fold it in 1/2 and make an overcast zigzag (see tutorial on this blog) to fasten the sides. You could also straight stitch the sides and then turn the bag RIGHT sides out.
The great thing about this is that you can either make your own cake mix (minus the wet ingredients) or you can just use your favorite premade mix (which is what I did here). If you don't want to get ramekins, you can use sandwich bags and make a fold over label that you staple on. I did that the first time. All of the mixes were names after my niece - Belle's Famous Sprinkle Cookie, etc. If they are just for your own kid to use at home, just buy a jar and keep the mix and a tablespoon in the jar and let them scoop and mix their own mix.