
Honestly, from the comments and slander made by the servants/nobles previously, i think its only because they are illegitimate. Born out of wedlock, the children of a commoner woman....considered to be a disgrace for the royal family. Which is just so effed up because its not the children's fault. They never asked to be born and they are faultless. Its a frustrating concept even in our world... the word 'bastard' being attached to someone because of someone else's actions. Never made sense.....sry for the rant

I like most of the answers to this, but also, like, I imagine the royal family is pretty coveted as marriage partners for the nobility, and imagine how much of a disgrace they would consider having their own lines marry into one they consider impure/illegitimate. Like, they probably can't refuse a kings order to marry one of them, so maybe they're trying to get rid of them before they could be a potential nuisance?
I think this is also a possibility, and I really can't wait to find out the answer to this as well
So why do they even care so much about killing these kids? Not only are they illegitimate they are the 4th and 5th child. Neither are in the running for the crown nor do they seem to have any particular duties the kingdom would need them for. Unless there’s some ‘magic’ reason that will be explained later for whatever reason.