If Parent work item is in complete/done state , no any child should create or attach to it
Accepted answer
EWM has a built in precondition "Strict Group Resolution" that provides this behavior: "Requires all children to be resolved before the parent can be resolved, and requires the parent to be open before a child can be added or re-opened."
And "All Children Resolved" that "Requires that a work item can only be resolved if all of its children are resolved."
If that is not enough, you would require a custom server extension.