I didn't spot them after I had initially finished the Werlyt storyline, but those Au Ra kids in Terncliff playing hide and seek do eventually reappear and find a fourth child.
...Except it seems they actually are the spirits of the departed orphans as the one resembling Alphonse remarks that his sister is with Father when asked where she's hiding, but after the other three leave, he turns towards where Allie and Gaius are standing and says she'll come play with them again much later.
I hope they keep putting more subtle little events like in future areas.
EDIT: Apparently, the kids only appear in the daytime. which would explain why I didn't see them.