I think that's part of the point of child posts -- not most of the point, but part of the point -- that they don't show up in the main directory. It's good for squirrelling away WIPs of finished drawings when both versions are on the booru, and such.
Child posts are still displayed normally when you search for any relevant tag, they're only undisplayed in the main directory. That's satisfactory to me, but I see what you're saying about newly-uploaded child posts. If you're worried people will miss one, perhaps just don't list it as a child post right away? Instead, leave it as a standalone image for a couple of days and make a note to come back and establish the parent-child relationship later. That way you know the regular members and anons got a chance to see it, yet ultimately the parent-child relationship does get declared.
That's the best I can suggest because, on the basis of comparable questions in the past, I doubt whether Pie can tweak an intrinsic mechanic of the booru like that. (Of course that's Pie's cue to come in and say "Easy!")