Open orphanage, raise assassins?
Positive spin "raising the next generation"
More realistic spin "indoctrinate the next generation"
Even outside of assassins, once these children go out into the world they become potential source of information whether they're trained that way or not.
Like imagine one of these children goes on to become a servant serving a noble, just keeping in contact with them will give a glimpse into the ongoings of that house.
Doesn't even have to be explicit "tell us what's going on" order, just put on a friendly spin like "we'll miss you, send letters to tell us you're doing fine" or such.