Strangely enough, if they were broadcasting their existence they'd no longer a secret order. They seem to be somewhat justified in trying to hide their existence, as in past times Templars were hunted and killed. According to Medb that's what led to her adopted human brother, who had Duren Dall at the time, to losing it and his life when he was killed by humans at the stake. Rightly or wrongly, I'm guessing there's a paranoia amongst the order that when other humans know about them they get killed off. I don't think either parent had any sort of malicious intent behind shielding their two kids from the order for as long as possible... both threats to the Grandmaster using them, and to keep them out of harm's way.
I could be completely wrong, but that's just my take on it.
Doesn't Linnae (spelling? grandmaster that stepped down) also say she lost her family to the order's secrets if you talk to her in the templar bar?