Could this sentence also mean "They are my happy fathers"? Maybe that's an unusual thing to say in English, but I don't understand why it couldn't be a possible translation.
You could try and flag your translation as a possible one. Possible translations have to be added by hand and no one thinks of all of them when making the sentences.
Yes, it could! It's easy to miss translations like this when adding sentences. Slowly but surely all acceptable translations are being added. This one is now accepted.
sorry, but no. "they are my happy fathers" would be "ñuhi kirini kepi issi"