What will happen to my cake if I don't cream the butter enough? I am preparing for a cake decorating party for some 8-year-olds and was testing a simple butter cake recipe.  

I made a small-ish batch of batter (just over a stick of butter, just under 1/2 lb of flour), but I have a large stand mixer and I don't think I really creamed the butter enough.  (This isn't the first time I've run into the issue of the large mixer not being ideal for small batches.)

The test cakes seemed to come out fine - they certainly aren't terrible - but I was wondering what, in general, happens to a cake when you're using the creaming method but don't really cream the butter enough?  Does the cake come out heavier?
The idea of creaming is definitely to incorporate some air into the fat-sugar mixture, which should give the final product a lighter texture. So perhaps your cake was a little heavier than it was supposed to be. Good creaming also helps distribute the fat well.

But maybe you managed to mix some more air in later in the recipe - certainly I imagine you could do better beating the batter with a stand mixer than I can by hand! And with a stand mixer I'm sure it was all mixed fine, too. So if the cakes weren't too heavy for you, it's not anything to worry about.