Don't really have much advise but for DS birthday I baked the cake the day before as well as made salads and other stuff that could be done the day before. So that on the day all I needed to do was ice the cake and make a couple of other things.
Have you got someone who could help you out? Two sets of hands are always better then one.
Oh and you could also right a list of everything that needs to be done so that you aren't stressing about forgetting something.
And relax and enjoy
. You will do great with the cakes, it is much easier then it sounds (coming from the one who just covered a cake with different coloured patches of icing). Oh another idea is to do a plan of each cake on paper and mark down what colour icing goes where and what decorations go where e.t.c.
Good luck and I am sure it will go fine.