Rachel -
I never saw this in Spain or Chile, but when I was in Mexico (Aguascalientes and Leon), I saw it at a lot of birthday parties.
I was told by one of the mothers that it is tradition to let the birthday boy or girl take the first bite of their cake. When the kid leans in to take a bite, you push his head in. It was really funny to watch! Some kids were excited for it, and others were a little apprehensive, but everyone laughed.
I have a few pictures...I will have to look for them. I am not sure if it is only a tradition in Mexico, or certain parts of Mexico, but that is the only place I saw it.