Probably if they are that disaffected with Judaism that they are mechalel shabbos, the parents are the problem and telling the will make it worse.
Kids perceive Hashem the way they think of their father.
If their father is fair, and understanding of their struggles, and understanding of their feelings- then that is how they perceive Hashem.
If their father is critical, insists they do what he wants regardless of what they want or how hard it is, equates minor infractions with major ones (because the “sin” is not listening to him), does not give leeway when things are hard, cares more about their behavior than their feelings- then that is how they perceive Hashem.
Now I ask you: if Hashem really was like the second type; would you want to keep shabbos?