Gliding
The /w/ and /y/ sounds are classified as "glides." Gliding is a phonological process typically affecting /r/ and /l/, which are classified as "liquids." It's probably safe to say that anyone who spends much time around Standard American English-speaking children has observed this process first-hand and can think of several children who pronounce /r/ and /l/ as /w/ (my right leg becomes my wight weg), or /l/ as /y/ (lemonade becomes yemonade). Less commonly, /r/ will be glided as /y/ (four becomes foy).