Fantastic Children was the victim of a very peculiar set of circumstances in the West.
For whatever reason, it was announced as licensed by Bandai before it even aired - a particularly rare thing to happen in 2004 unless your show was one of the various Geneon co-productions. Furthermore, no-one really had a clue what it was - if I remember rightly the show's website wasn't up and running yet, or at least there was just generally very little awareness that the show even existed. There was a fair amount of discussion as to whether it was a retitling of the then-recent Shin Megami Tensei Devil Children anime for the Western market! As pre-licensing was relatively rare in those days there was much more of an attitude in the circles of fandom I moved in that such shows were not to be touched by most fansubbers, so FChildren quickly fell into obscurity as a result.
Its promotion in Japan, as I remember, was heavily based around two things that are essentially meaningless to Western audiences - that Nippon Animation were animating it (in particular because they also did Future Boy Conan) and Takashi Nakamura was directing (the Ghibli link was played up here).
Regardless of the quality of the show, its old-school character designs combined with the marketing mess above rather doomed it to obscurity - an enormous shame, really.
The character design coupled with the lack of action and lack of any kind of fanservice was a death sentence.
From the New World kinda suffered from the same problems, people complaining the first half was too slow and the DVD sold next to nothing.