This just question just popped into my head (literally, like five seconds ago).
Is there anything in the Planescape materials that covers what happens if a Petitioner has a child, or even if they can have them? Things that use to be petitioners like fiends can have them of course, so I assume that Petitioners can as well but I coudl be wrong.
If a petitioner does have a child, is the child a petitioner as well, or is it like a "normal" planar/prime or is the kid "brainwashed" into automatically being devout to whatever deity/ideals the petitioner represents?
I know this concept is kind of weird with some of the "less humanoid" petitioners like those of limbo, but I was thinking of the humanoid ones (elves, orcs, goblins, etc.,) in this case.
Yeah, I know fiends aren't petitioners anymore once the pass the beginning stages (well, Yugoloths are never petitioners . . .).
That soul child thing was exactly what made me think of this. I was wondering how petitioners of certain powers could possibly exemplify the qualities of their deity without being able to have young.
As a general rule, I don't believe so. But I would certainly say that some powers may enable their petitioners to have children who grow old and such. The process for it happening would likely be different, however, and very rare.
But it's something interesting to answer in the books 
Fiends and the like aren't petitioners anymore.
Manes, lemures, and larvae are sexless, while I think the ghostly shades of the Gray Waste and lantern archons are incorporeal. I assume that petitioners can't have young - they rely on the living to replenish their numbers (by dying).
A big exception is the soul child.