If you ask, respectfully and politely; and if you can persuade the parent -- and siblings, if any -- that you will not be harming any of their family members' reputations -- then you may be able to get the permission you really should have. Think beyond just the legal aspects here; being considerate and thoughtful and polite, can help all deal with what was a tragedy.
If they refuse permission, then you should change the name(s) -- and in a fashion that breaks the link between the real and fictitious characters.
Or you might end up spending a deal of money on attorneys (trying to negotiate permissions, dealing with emotion-driven lawsuits, etc.) which is not a sensible or cost-effective path.
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The easiest thing to do is change the names and enough of the identifying details of the real people your work is based on to avoid any problem with the real person, and include a disclaimer noting that your screenplay is a work of fiction, and any resemblance to anyone living or dead is a coincidence.
Once your screenplay's done, see your own IP lawyer to have to registered for a copyight and "vetted" for any problems (besides this one that you already know about).
As my colleague has noted, it's better to be safe and get a release from anyone and their family members who could claim a violation of their privacy and/or publicity rights, but that ma not be necessary with proper vetting.
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