My son is still young enough to be practicing his pronunciation. He has trouble (not surprisingly) with th in particular. I have modeled the correct mouth shape for him but it still takes him a lot of effort to get the sound right. In regular speech, he just slurs the sounds into f or s depending on the word.
With his l sounds, we had a lot of success by singing "Skip to My Lou" a lot. I can't think of any children's songs that have frequent th sounds though.
Is there a current, accepted, standard technique to help him practice both the voiced and unvoiced th (which preferably don't involve sitting and saying "th, th, th" for a long time)?