cat's out of the bag.
I won't notify you, sarsak, but the most probable reason is that it'd be harder to gain custody of both of them. the mom probably wanted one of the kids so the dad would send money/she'd have a connection to the father who makes a lot of money. she probably wouldn't go down without a fight, so it was understood by both children that it'd be easier for one of them to suffer with the mom rather than both of them potentially suffering with the mom. there have been cases where jurors are biased towards mothers in custody cases, at least in the US. not sure for Korea lol