For a baby doing the "razz" click on this video

blowing raspberries
 

Blowing a raspberry or making a Bronx cheer is to make a noise made to signify derision (and/or silliness), made by sticking out the tongue between the lips and blowing to make a sound reminiscent of flatulence.  

In the terminology of phonetics, this sound does not appear to have an official name, but might be characterized as a linguolabialtrill.  It is never used in human language phonemically (i.e., to be used as a building block of words), but it is widely used across human cultures as well as by other primates.