"Their father is in America."
Translation:उनके पिता अमेरिका में हैं।
If you use उनका, you are treating पिता as singular so you need to conjugate with है.
Note: पिता is usually treated as a 'formal' word by itself so it is rare to use it as a singular word, even when the person speaking is older than the father. बाप is a less polite alternative which you will sometimes see in the singular form.