One has a head spot, so definitely a boy since I have barred pullets but no barred or cuckoo cock(erel)s. The other doesn't, so she's definitely a she because she hatched from an egg I marked as BPR (the barred boy chick hatched from an egg I actually thought was probably laid by one of the Light Sussex, but clearly not!).
I remember collecting the egg the black girl chick hatched from, right after one of my BPR pullets emerged from a nest box shouting about the egg she'd just laid. Shetlands should lay green eggs, so for that to be a Shetland egg you'd need:
Another of the Shetlands to have started laying without me realising (the confirmed layers, I know for a fact all lay green)
AND that Shetland to be laying the same colour as the Barred Rocks, not green like she should
AND the Rock pullet that I thought laid the egg had actually laid a "phantom egg" since there was only one egg in the nest when I collected it (this did actually happen recently but it was obvious that time because the only egg in the nest after she left was green)
So, TLDR: if that's a Shetland cross chick I'll eat my hat, muddy Fayoumi footprints and all
The yellow chick DEFINITELY hatched from a green egg, so it can only have been laid by one of the Shetlands. I have a red barnyard mix who lays pale olive, but a different enough shade that it's easy to tell at a glance whose eggs are whose. She sometimes lays slightly more of a pale sage/grey green, but never the green I get from the Shetlands. I do have a wheaten Shetland but I'm fairly sure she hasn't started laying yet, and even more doubtful that she was already laying decent-sized eggs three weeks ago. Could at least one of the Shetlands be something like E/E^Wh or E^R/E^Wh (would that fit the prime suspect pullet in the pics I posted?) and have passed on Wheaten, rather than Extended Black or Birchen, which was then diluted by dad's Silver?
Edit - I wouldn't like to say for sure until s/he feathers out, but the yellow chick also looks like it might be slightly crested, which again could only have come from the Shetlands.