I took an entire regular cake mix, put it in my food processor, and added 1/3 cup of solid shortening (Crisco). I whirled that around until it was like coarse meal. This acts as the oil in a regularly-baked box mix. Then, I packaged them up in 1/4 cup portions, using those wonderful little "snack size" ziplocks.
When the youngun is ready to bake, take 1 egg, and beat it up with a scant 1/4 cup of water.