How cute!
With a Black australorp (E/E) x Buff orpington (usually E^Wh/E^Wh) you would expect your roo to genetically be E/E^Wh, which should be an all-black chicken. The fact that his silver is showing makes me think that there's something else in there, that the black australorp parent had some sort of recessive gene hiding (maybe it was E/E^R, or if there is some wyandotte in there, E/e^b), and that your roo is maybe E^R/E^Wh, or even E^Wh/e^b with lots of melanizing genes causing the black to appear on the wheaten.
Since the parents are hybrids/mixes themselves carrying recessive e alleles (e is the allele that controls the color black and how much black appears on a chicken), it's not surprising that the same mix could produce a bunch of different patterned chicks.
The gray-ish chick to the left has coloration consistent with the dominant E (extended black) gene as expressed in chicks, which it would have gotten from its sex-linked mom (E/E^Wh). The darker chick on the right is hard to judge without seeing its head/chest, but could indicate a birchen (E^R) gene inherited from dad expressing dominance over a E^Wh inherited from mom. All that to say, each of these chicks could reasonably come from your roo x a sex-link.
I was less inclined to think the white/light yellow chick comes from Lavender simply because she seems to have a solid black base (E) that has been diluted by either blue (Bl/b) or lavender (lav/lav). That said, she could be hiding a recessive e gene in there too, so I can't totally rule out that she's the mom of the chick, either!
