The police going to ask them in the first place was a wrong decision. A family minding their own business doing nothing wrong should not be burdened with someone showing up at their door saying "we think that you're the children of a murder rapist. If you don't help us that's definitely what we're going to conclude. If you do help us there's a chance that we'll conclude it anyway, but you never know. He's dead now so we have no requirement to overcome a legal defense. What say you?"

That's known as a devil's bargain.

> Maybe their reaction to learning

A more likely outcome is irreparable trauma.