Can anyone please advise me as to how I can put a fondant plaque with a birthday message on it, on top of a cake covered in fresh whipped cream without having the fondant fall apart? Thanks!

You cut a piece of parchment or wax paper the size off the plaque and place it between the cake and plaque as a backing 

Back it with chocolate or set in into a chocolate base plate so that the fondant doesn't touch the whipped cream.

You could also opt for a pliable product like modelling chocolate/princess paste that won't melt in the whipped cream.

Thanks for the responses guys. I’m thinking of using parchment paper and chocolate. My plan is to cut a piece of parchment paper, put chocolate on the back of it and then put the fondant on the non coated side. If anyone think this is a bad idea, let me know! :)

You don't need to put paper between the chocolate and the fondant, just melt the chocolate, let it cool until it is not set but not very warm to use it and male sure your fondant plaque is well set/dried before you put the chocolate on it.

Thanks bubs1stbirthday!