Geoffrey,
This appears to be an automated profile created by the software when you added siblings. The software needs an 'anchor' to be able to establish relationships such as siblings-so it creates one automatically.
Since the children's profiles are not open for us to see to be able to help out, there are a few things you can do:
1. subtract about 20 years from the earliest born child's birth year and plug in that for Unknown's 'uncertain' birth date.
2. add 1 year to the latest born child's birth year and plug in that for Unknown's 'died after' death date.
3. Plug in the latest born child's birth location as Unknown's death location.
4. Add some text to the biography to explain the situation...something like "Unknown is only known through his children's records. All dates/locations are speculative."
5. As you are the person most knowledgeable about this family, use your best thoughts to extrapolate as much as you possibly can about Unknown. Any narrowing down or additional tidbit is useful for others looking at this profile in the future.