Who said that Trigon had to be sleeper agent? He knows how to act like a reasonable father, and he doesn't want a war started on his doorstep. He might even know how to change a diaper.
There is also the fact, that if he does nothing to taint the child, he will drive three generations of Aes crazy as they try to figure out what he did to the child.
There is also the fact, that if he does nothing to taint the child, he will drive three generations of Aes crazy as they try to figure out what he did to the child.