There will always be perverts sending nudes to kids one way or another. Where I grew (before the internet) we've had them arrested near schools.
Pretending the world is all unicorns and rainbows is not the way to raise your children IMO - the hard truth is going to hit them sooner or later. Talk to them, treat them as young adults, and explain what is good and what is not, and why.
Fun story: I'm into crossfit semi-professionally and I've recently been very pleased to hear my 5 y.o. telling her friends that they should not eat that much fries and coke if they want to stay healthy and suggested they try fish and chicken instead.