"Their father is in the USA."
Translation:उनके पिता अमेरिका में हैं।
पिता, by default, takes plural verbs and adjectives as do words like माँ, दादा, राष्ट्रपति, गुरु, etc. This is a case of what's called the honorific plural.
I used बाप instead of पिता (typing the answer), but it didn't recognise it as a valid translation