So if Cara told him his Confessor son would unleash hell I'm positive he would have listened. Maybe - it's hard to say what he would've done next.

But I think the reason she doesn't tell him is how angry she is about her son - she doesn't know Darken ordered him to be killed, but she does know he was nowhere to be found in the future. And she probably always figured someday he'd come into his own as Darken's heir, meaning Nicholas being Lord Rahl over her son would make her much angrier than Darken marrying Kahlan... at least, that's my theory ;)

Or maybe a little bit more Darken/Kahlan romance in Princess Rahl... ;)) One tries :D