The ones that speculated he was going to play a young Jack were the ones that wrote the article. Disney hasn't said anything about how Dominic will come in to play. I think it was PLF that said that his name wasn't posted in IMBD because it would give away to much. So I really doubt he is going to play a young Jack.
Regardless of who the father of the child is, I don't think it will be a 'secret' love child. That's about as far out there as any of the men becoming gay and taking each other for lovers.
So if Will and Elizabeth do get married, I doubt that there will be any doubt about who the father is. If Elizabeth and Jack get together, the child will be theirs.
Of course, I've suspected that there is a child of Jack's somewhere out there, but I don't know for sure. 😄