In the OT outside world, everyone would know that the O6 (more or less) have got on a plane, and disappeared in almost identical circumstances to the Oceanic 815 crash. This would be an incredibly massive news story. (I have often wondered why no one has talked about this.)
However, both Je Yeon's and Aaron's grandparents would be very easy prey to Widmore who could weave stories about an island, and prove to them that the parents are still very much alive. In addition, we know that Widmore had connections with Paik in the past of the OT, and surely he must have had tags on Ben and his dodgy lawyer, so it would not have been too difficult to find out about Aaron (and lo & behold, the same lawyer acted for Ben and Carole Littleton - what a coincidence!). If Widmore has not followed this up, then Widmore would be missing a trick, which is totally unlike Widmore! Whether the children have actually been 'loaned' or 'kidnapped' is irrelevant. The storyline has always demanded that Aaron return to the island, and this is just about the last opportunity for him to to do so, unless Desmond & Penny happen to come sailing by with their floating creche.
In the case of Je Yeon, the narrative went to enormous efforts to tell a story about Jin's impotence, then have Sun become pregnant despite the island's pregancy issues, and ensure Je Yeon got born - but not on the island. Jin/Sun then got separated, giving Sun had a reason to come back, but leaving Je Yeon in the outside world. Since then, Je Yeon has been on the back burner. How many times has anyone in the story mentioned him? Has Sun been pining after him? No? I smell a rat. The story somehow demanded that Je Yeon was born, but then leaves him behind, and virtually ignores his existence, so that the audience casually forgets about him. The family is separated: Jin is with MiB, Sun is with Jacob, and we don't know who the candidate KWON is supposed to be (Ilana is confused as to whether it is Sun or Jin, but never for a moment considers Je Yeon); come on, how many clues do we need? We still do not know whose side Widmore is on: if he has got Je Yeon, and if she is a candidate, that becomes a pretty huge deal. Even if Je Yeon is not a candidate, it's a bloody good red herring to toss into the mix at this stage.