grammar - Using the idiom "peace of mind"

I'm not sure if there's a clear answer to your question. It might be easier to solve your problem if you use complete sentences. Here are some examples...

1. Knowing that her children were safe gave her peace of mind.




2. She had peace of mind knowing her children were safe.

[I'm not sure if this sentence is grammatically correct, but I think most would judge it OK.]

3. She had the peace of mind that comes with knowing one's children are
   safe.

I think peace of mind IN knowing might be OK in certain situations, but it sounds a little awkward to me.

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