Hi, I'm making my sons first birthday cake soon and it'll be the first time I've used fondant. Do I NEED to put a buttercream crumb coat under the fondant and if I do, how do I know how much buttercream I'll need?

Yes, anything that's under fondant will telegraph  through, so a little bump will look like a big bump on the fondant. Just do a thin coat of icing to smooth out the surface, you don't need to do as much as if ti was just iced with buttercream by itself.

You need something for the fondant to adhere to.  I would put a crumb coat on the cake.  As for how much, just that, a thin crumb coat.  A thick buttercream layer will do two things:  1.sag under the weight of the fondant thus causing you fondant to sag, 2. Break down the fondant from the moisture.  How long the second takes depends on the thickness of your fondant.

Another option is to coat your cake using a whipped ganache, let it set and apply a very very thin coat of apricot preserves to the ganache as a adhesive for the fondanant.

Hope this helps.

Herekittykitty mentioned what I seldom see any more - apricot jam.  Why apricot I don't know.  I would say any light flavored, clear jam/jelly would work.  Think apple, peach, pineapple.   Just make sure it does not have any bits or pieces of fruit in it.

Oooppps - can't change a post but I just thought I bet even piping gel would work.  Just put a thin coating on.  You need the cake surface to be sticky, not a 'taste coating'.  

Yup, that's why I like apricot (jam not.preserves, oops - thanks Kate ladies), no noticeable flavor and nice and sticky.  

Kakeladi.  Sorry auto correct and I can do change the post.