"My dad's brother is my uncle."
Translation:Min pappas bror är min farbror.
Pappa is not child language, however it could be seen as more informal than far. The same goes for mamma and mor. I'm don't know all the nuances of English synonyms for father, but maybe if you could think more along the lines of papa or pa in English? More informal than father, but something adults say.
Just as an example, I think Crown Princess Victoria only calls her father pappa in interviews, and it is not some kind of affectation, many adults do the same. Others say far, which again seems to be more of a personal preference (or how you've heard your own parents call their parents), than people being more respectful, formal or some such, at least nowadays.
At least in American English, it's very rare to hear anyone refer to their father as "papa". It would be considered very childish or old-timey. Pa is also very uncommon and comes off as someone very rural would say.
In American English, the most colloquial term for "father" is dad. "Pops" is another colloquial way of referring to your father but it is far less common. I'm willing to bet 90%+ of Americans refer to their father as "dad". Young children often refer to their father as "daddy"
Same thing for mother. "Ma/mama" is old-fashioned while "mom" is considered the common colloquial term. Young children often say "mommy".