How fun. I made my first real decorated myself cake for DD1 2nd birthday. It was a cow and the theme was MOO MOO guess who's TWO
I made it all the night before and used both Fondant and butter icing and found that Fondant is really easy to work with.
I had white fondant and coloured it myself. I would not freeze anything and I would do it the night before as you don't want to be rushing it on the day and if you need to fix anything then you have the next day to do it.
I would say make sure you are not disturbed so you don't lose focus.

I used packet mixes for this cake, I love the Greens packets. I usually make cakes myself but just thought I would go easy on the actual cake making and focus on decorating.

Goodluck. It is the best feeling when you finish after so much work has been put into it and it looks great.