Changing behaviours takes time and involves others. Educating young competitive swimmers on the benefits of eating a nutritious meal three times a day - with suitability healthy snacks too - is one thing, getting them to change their behaviour is quite another.
Pushing it to the post-grad level here is some further reading:
Two frameworks that I find really helpful is the Stages of Change model...
The COM-B model of behaviour change
Think about the stage they are at.
Think about their parents/guardians.