Probably there is nothing seriously wrong, and her substitutions may correct themselves in time with little effort on your part and with no formal therapy. Over-correction causes frustration, and it also causes anxiety. So, whatever you do, watch for signs of that in your daughter and pull back if any evidence presents itself.
The two substitutions you mention are about the most common that one sees in young children. Think about it. When a comedian wants to imitate “baby talk” he says something like: “I fink (‘f’ for ‘th’) I saw a wabbit wun (‘w’ for ‘r’).” Children usually correct these errors on their own, but a little work on your part wouldn’t hurt.
Without making a big deal of it and causing anxiety, have her sit for a few minutes facing you and say, “Watch my mouth when I say, “Thumb.” To make the “th” sound you have to part your lips and put your tongue under your upper front teeth. Have her make just the mouth movements without any sound a few times, maybe using a mirror, until you are certain she can make them.
Then do the same thing with the “f” sound. For that the upper teeth have to touch the lower lip. Then say “thumb” and have her imitate it, exaggerating the placement of your tongue. The term “thumbs up” is so popular now you can say it several times and have her imitate both the gesture and the sound. Another simple suggestion is to put a Band-Aid® on one thumb, which kids love to wear, and ask from time to time, “Where’s your Band-Aid®?” If she says, “On my fum,” face her again and remind her how to make the “th” sound. Find some simple “th” words and let her work on them.
The way to correct a “w” for “r” substitution is not as easy to demonstrate visually because the “r” is made in the back of the mouth. Not so for the “w,” which involves pursing the lips and expelling air. Play with both of those sounds a little bit without extending them to words. When she imitates correctly say, “That’s right,” prolonging the “r” sound. Then give her a few more “r” words, choosing simple objects and words she definitely knows. Have her pronounce them, then ask her to tell you whether she was “right” or “wrong.” Overall, it sounds to me as though you are doing all the right things. I’m just suggesting that you might do a few more.
Our parenting advice is given as suggestions only. We recommend you also consult your healthcare provider, and urge you to contact them immediately if your question is urgent or about a medical condition.