Along with children born 300 years after their father, I always presume time travel was involved. Still looking for that time machine to hitch a ride in myself... I know it's got to be out there somewhere.
With reference to Weldelphus... not sure which father you were referring to. It looks like a complete mess in this area. And I'm personally not convinced that Weldelphus ever existed.
There is a project, though, that is dedicated to cleaning up a proposed early lineage of the Franks, which may eventually touch on this lineage:
Would suggest checking that out.