Translation:My parents are interested in playing tennis.
"verbs like gustar" only take the singular and plural prefix, and they refer to the object, not the subject. You will only ever see gusta/gustan and interesa/interesan https://www.spanishdict.com/guide/verbs-like-gustar
That’s not quite true: the person you like, or who interests you, can also appear in second person:
This is soooo confusing. The literal translation is "In my parents them interested to play the tennis.
Sometimes the structure of sentences in other languages is very different. I find it helps to look up a bit of grammar.
I think because you're just skipping the word "jugar" if you translate it that way.
That construction sounds somewhat weird. Usually you say "to be interested in [something]", or "to be interested [enough] to [do something]", but not a mix of these.
Jugar can translate as "play", "to play", or "playing", depending on the situation.