All I can think of at this point is that if Astro and Atlas are brothers.... then Astro and Nicky are siblings as well, or at the very least half siblings.
I guess robots wouldn't encounter the equivalent of the genetic problems humans would in this situation, although one could argue that it's safer to have two different computer systems in case one has a bug.
Am I starting to reason a bit like fafner?
That said, I would think the kids would probably be the exact same size of their parents. They would probably be very well behaved, obedient and courageous. I'm sure Nicky would be more than happy to spare them the bomb she had inside, but as for other powers, I don't know. I know Uran flies in the 1960 series at some point but she's still not like Astro and I could never understand why one wouldn't want to make her a 2nd Astro (other than the fact that it wouldn't be a good idea to have multiple heroes in a manga, and if I try I can only imagine what damage a newborn Uran would have made with Astro's powers right from the start!).
This being Astro's world though, I would say that the children wouldn't be raised with Astro's powers, at least not while they are still young. I would think Astro would be worried if his children followed his path as it's a dangerous one!
Boy or girl? I don't know, Astro is a boy but I've always had an easy time identifying myself with him so I wouldn't think he'd have anything against having a girl. I don't think it would matter to them. I would also think they'd want more than just one child so there could be boys and girls.