I think "They are happy fathers and mothers" would be "Kirini kepi muñī issi"?
can't we say : they are happy mothers and fathers. instead which flows better
It looks like the order of words is important: "and" is in the word of "mothers", so it is "mothers-and".
its not about what flows better. Also they are differemt sentences. "they are happy mothers and fathers" is "Kirini kepi muñī issi" its the difference between passive form and plain form. Its the same concept as "the boy grabed the rock" and "the rock was grabbed by the boy" in one sentence the boy is the subject, in the other the rock is the subject even though it tells the same story. So in one sentence the mothers and fathers are the subject, and in the other the happy mothers and fathers. It doesnt change much much, but however small the change is change none the less. One more small detail is your english sentence does not specify if both the mothers and the fathers are happy or just the mothers.
Why is the i in kepi sometimes enloged and sometimes not? Is it always the second noun's final vocal that is stretched? (compare Hontesse kepī kirini issi)