Yesterday I was talking to a little boy who mispronounced a word. It was a very common speech difficulty for a boy his age. I think he used a "k" sound instead of "tr" in a word. (Maybe it was "truck".) I repeated the word back to him in a sentence and pronounced the word very clearly for him. He watched my mouth when I said it and then said, "Yeah, truck," and said the word much better that time.
I find that works very well with kids learning to pronounce words they have difficulty with. Whether it's a regular lisp, or a fancy word, if I repeat the word slowly and carefully, even sometimes saying "Watch my mouth," they often correct the word pretty easily. Since they can't read, they're trying to mimic words they hear that go past quickly, and sometimes the best they can do is an approximation. If we slow down for them and let them see our mouths, they see how the lips and tongue are forming the word and saying it correctly is much easier. Adults figure it out by looking at the spelling, but that doesn't work for a two year old.
That's also why I'm a firm believer in talking to our kids in a grown-up voice, using complete sentences and modeling language the way we want them to learn it. Baby talk is for puppies and kittens. I don't think we want Emily at age five to go up to the kindergarten teacher and say "Widdle Emiwy needs to go potty."