Well it is in beta so maybe it just needs to be added. Just keep reporting any mistakes like that you find and it should be sorted.
No. "You are all fathers" is different. I confirmed this with other Duolingo contributors who work on courses that also have a second person plural. They don't accept "you are all" for second person plural, and neither does High Valyrian.
You all are fathers = Kepi iksāt ,You are all fathers = Kepir iksāt , Im not certain but I think that is this
"You all are fathers" is correct, and "You are all fathers" is incorrect. "iksāt" means "You all are fathers", not "You are all fathers".
The green correct screen says 'You are fathers' with an extra space between 'are' and 'fathers'. Nothing major, but it looks kind of sloppy and I thought I'd mention it.