50% should have 5 toes if one parent was split polydactyl/not, and the other was pure for not-polydactyl. Because the 5 toed parent would give the gene to half their offspring.
I disagree. Mixes are not common in stores that buy chicks from hatcheries, so probably not Silkie mixes.
And they look more like Cochins than Silkies. You say the beak color is off for Cochin--but the leg color (light), the number of toes (4), the comb (single), and the crest (missing) are wrong for Silkies. Aside from the feather texture, which looks more normal than silkie to me. So they might be poor quality Cochins, but they would be much worse as Silkies.