Q: My copyright co-owner died. Can his children stop my novel based on the treatment. Do they own 50% of novel

A: You've raised a lot of issues in your question and a comprehensive answer can't really be provided without more details. For the following general overview of the law, I'll assume:

(a) there are no written agreements between you and the WW2 vet or the WW2 vet's family;

(b) the treatment was never intended to stand on its own as a literary work, that is, that you and the WW2 vet both intended that the treatment would be subsequently adapted into another form;

(c) that you registered the work with the U.S. Copyright Office as joint authors.

Section 101 of Title 17 defines a "joint work" as "a work prepared by two or more authors with the intention that their contributions be merged into inseparable or interdependent parts of a unitary whole." Each owner of a joint work is a tenant in common with an equal and undivided ownership share. So, when a joint work is created, each author is simultaneously treated as if they owned 100% of the copyright in that work for the purposes of what they may do with the work. Each owner of a joint work is free to exploit the work without the consent of the other owners, but must account for the profits of that exploitation by splitting them equally among all of the joint owners. This exploitation right applies to NONexclusive licenses.

Now, things get complex pretty quickly in trying to determine if new derivative work has been created. Absent a written agreement between the joint owners of the underlying work, only a court can determine whether new derivative work has been created by an author:

If one author modifies a joint work to create a derivative work, that is, when sufficient original content is added to an underlying work, the result is a new derivative work, authored by the person who added the original content (this new author can be someone totally unrelated to the original authors of the underlying work). The authors of the underlying work do not share authorship with the author of the derivative work. The original joint authors intended to merge their contributions to create the original joint work, but their intent is not deemed to carry over to a derivative work to which they did not contribute. This is difficult to apply when it's not clear whether the underlying work and the derivative work are truly separate works.

An attorney who has the opportunity to review all of the associated documentation with your case will be able to give you clearer answers to your specific questions. Your post also raises some concerns beyond copyright law. If you're going to go ahead and publish your work without a written agreement with the WW2 vet's family, you'll also want to ask your attorney about your state's personality rights and libel laws. Generally the dead cannot be defamed and truth is an absolute defense to defamation. However, that does not necessarily prevent the surviving family members from filing a lawsuit for claims which can be related to defamation claims, like invasion of privacy, intentional infliction of emotional distress, or injurious falsehood.