Hi I'm a little bit of a newbie/beginner to all this. I make cakes but just for me and my extended family. 

I want to make the rose buttercream cake (the one that the roses are piped onto the cake) for my daughters cake smash.

can I make and decorate the cake in advance and somehow freeze it without it getting ruined, soggy and the frosting still staying soft when defrosted?

thank you so much sorry if this isn't such an "advanced" question

I have frozen one for about a week before.  I just wrapped it in plastic wrap after it was frozen solid (so the frosting wouldn't get smashed).  I took the plastic wrap off and thawed it in a cake box in the fridge overnight.  It was as good as fresh made.

Personally I find that the all shortening buttercream gets a bit dry and crumbly once it has been frozen.  You are better off with a buttercream containing at least half butter. 

The meringue based frostings would be good for what you want.

I totally agree with squirellycakes - my recipe has about 3/4 butter to 1/4 shortening.