I don't think we'll be able to have kids with Varena or Ryn until the Epilogue (assuming there is one) because we'd have to marry them first, otherwise the children would be illegitimate/bastards, which seems to be frowned upon. I know you can propose to Varena right now, but no actual wedding happens. Same thing with Ryn giving you the crown in Dracia.
I would very much love to be proven wrong, though.
As for who's more likely, I'm thinkin' it might be a side character. A quest to fix Lusamine's infertility, perhaps?