Since prevocalic /r/ misproductions are often characterized by gliding or
w/r substitution that is an issue that should cease at around age five.
Many times it remains an issue beyond the preschool years. I have seen many SLP’s administer intervention for prevocalic /r/ by instructing students produce an er in front of the prevocalic /r/ sound as in the words red, becomes errrr red. Unfortunately, this becomes habitual for students and does not sound natural in conversational speech. The students are continuing to round their lips as they did in the w/r/ substitution.
What I have recently found to be more effective in my interventions with younger and older students is auditory discrimination of w/r words. I produce two words, one with /w/ and the other with / /r/ as in wed vs. red. I hold my mouth behind a piece of paper and ask the student to listen carefully to see if I have students raise their hand every time they hear an /r/ sound and keep the hand down if I say a word that begins with /w/. Oftentimes, students cannot hear the difference and so this is an important foundation for beginning to hear and correct error patterns. The exercise is fun for students, yet effective at the same time.
I often times have parents do the auditory bombardment exercise at home for homework using the Entire World of R Activity Books