I am a tutor, and I am often working with little kids (K-3rd grades) on their phonics.
One thing I have always been curious about is why they often have the “Elmer Fudd”
thing going with their r’s and l’s where they say the “w” sound instead. Doing some
impromptu testing, checking what my lips and tongue are doing for all 3 sounds,
‘W" requires a definite rounding of the lips which doesn’t occur at all with the other
two sounds. They usually don’t have any problem with other sounds-so why is this
one such a problem? I typically don’t count it “wrong” when they do it. Usually kids
outgrow it sooner or later (which leaves unanswered why ol’ Elmer never did).
Having sounded like Mr. Fudd myself for nearly all my life, I can shed a little light on the subject.
Start with the “L” sound, or more precisely, “Lah.” It’s primarily produced by the tongue touching the top of the mouth, right behind the upper teeth. Keep the tongue on the floor of the mouth – a lazy tongue – and it becomes “Wah.”
As for the “Rrrr” sound, the muscles around the lower lip are drawn more tightly than those around the upper lip. Relax those lower lip muscles a little, and the sound morphs into “Wuh.”
This might seem very simple to correct. It’s not – just ask Tom Brokaw or Barbara Walters.
But whatever you do… fwee Wodewick! 
I think both the “Lah” sound and the “Rrr” turning in “wah” are the result of the lazy tongue. Try reading a paragraph with the tip of your tongue held against the back of your two front lower teeth.
I can’t really answer the details on why, but at 38 (or, as I like to say, 0x26) I still have to pay attention to my R’s. I have to do a half-smile type thing to get the sound right (I don’t know if that’s normal or not). A lazy “R” doesn’t have the same feel as a deliberate “W” and sounds slightly different. Since I don’t know how anyone else forms the “R” sound I can’t tell you how to simulate what Fudd is exaggerating, but it’s not a “W”