Tosh's genes + Ianto's genes = a child that would take over the freakin' world. Trufax. Which was something the Toshiko-muse took into consideration when she picked Ianto: He's insanely smart, he's good looking, she loves him intensely, he'd make a great father and she really did want to give him -- and Jack -- a child (she probably also wanted Owen's but probably still would have picked Ianto's anyway, because Owen's not really 'daddy' material).

Ianto -- mostly -- didn't tell the others because it was sort of understood that it was either his or Jack's (especially as she got older and kept the blue eyes), but if you say it out loud it puts a different spin on things, and he didn't want to alienate Gwen or Owen.

I'm glad I added that too, 'cause at first it was straight from birth to her going away, and I needed to show how she'd become the kid they sent to Japan. I imagine that she was close to the rest of the team, but Jack and Ianto were the favorites because they babysat the most.