Could you use cream cheese frosting to replace buttercream. I don't have the Ingredients from buttercream right now and cant run out and get any. For a fondant cake by the way.

Depends. If this is for that lactose intolerant kid, then nope.

This thread reports that the cream cheese breaks down the fondant: http://www.cakecentral.com/forum/t/648991/what-happened-to-my-cake

This one is the test run.