So, that one *BA-DUMP* during the "You Have Us" update, when Lil Pup attacked Sans, was that a reaction from Frisk trying to pull Sans' soul out of his body? Seeing how Chara took Asriel's without actually turning him to ash, I thought back to that. I guess that the 'assassins' are children so they can become close to special monsters and take their souls easier?
YES YOU ARE CORRECT MY CLEVER FRIEND!!!
As to Asriel not turning to dust immediately, it’s because he’s a boss monster and they tend to last longer after their death (if you remember the game).
So yes. These children were prepared by humans as sleeper agents; ready to be planted in the most important locations/monster’s homes so that another war would help trigger them and bring devastating impacts on monsters.