I believe this is a sign of a hearing problem. Some children are unable to hear a strong difference between certain sounds so, to them, they are speaking the same as everyone else. It used to be that those children would be identified and given special help to allow them to hear better. I remember having eye and ear tests at school. Maybe this has stopped now?

The tragedy is that this is a huge dividing factor in the UK. In all the places I've worked the vast majority of people use the distinct "th" sounds. Use of "f" and "v" sounds is, to use the current terminology, a "red flag".