Eirika's joint ending with Tana also mentions each of them having children.

IIRC, none of Ephraim's endings mention children. It may be significant.

look at it form all sides and understand that getting heirs might be a kingly duty as well.

Let's play Devil's Advocate-- is it at all possible, given what played out in Grado-- plus the number of possible royal/commoner hookups postwar-- that there might be a reduced emphasis on bloodline kingship, leading more to "elected" kings or adopted heirs or something else that doesn't leave massive power in the hands of frail and unstable biological heirs?

There's another means of getting around the heir issue, anyway.