It's not healthy to raise kids in a protective bubble
Or: The risks don't justify keeping every child home. And therefore the only reason a hypothetically traumatized mother keeps a child home is that the mother herself needs it to feel safe.
Which means it's not about the child's well-being at all, it's about the parent's.
My logic probably sounds unsympathetic, but I'm not. I understand. Trauma doesn't bug you for days or weeks and then leave; it rearranges your life.
It's unrealistic to expect its victims to shed every memory of it, especially during the profound and emotional experience of child-rearing.
But there has to be compassion for the children, too.
For one thing, sometimes home is the problem. So much -- too much -- trauma comes from within families. You said it yourself, in a way. You refer to possible abuse by "a friend's brother or father or grandfather" -- ergo, the friend in your hypothetical has a predator in the house herself. A culture of permitting kids to visit other houses, within responsible limits that reflect each family's needs, doesn't only introduce potential harm; it can rescue kids, too. It can give an abused child temporary refuge, and chances to know how healthy environments feel.
It can also just broaden typical kids' horizons.
Plus -- parents aren't creating something on their own, of their own and that only they control. Child-rearing isn't sculpting. It's loving, protecting, teaching and ultimately releasing a fully realized human whose experiences from the beginning are her own. Emma deserves a chance at the childhood her friends are having when they come to her house. One of nuance, due diligence and trust.
And they all deserve a definition of "safe" that protects them not just from predation, but also from emotional stunting, from the joy-warping effects of irrational fear, and from the risk-seeking rebellions often launched by teens and young adults who've been raised like veal.
So, parents who have been traumatized owe it to their kids to manage their own emotional health sufficiently, in treatment if necessary, to accept some calculated risk on behalf of their kids.