Ivan and
Claudia sound very natural together. I can really see them already as siblings. :-)
Your lists of mns for
Ivan and
Claudia are so good I can't pick one out above the others. If forced, then
Ivan Septimus and
Claudia Alice would be it. You can't go wrong with any of them, though.
You two are just the sort of name nerds I like to see reproducing. :-D