Learn Swedish: Pojken är lat, medan barnet är slött. - The boy is lazy, whereas the child is lethargic.

The adjective lat (lazy) is defective, meaning it simply does not have a neuter singular form. The word latt does not exist in Swedish. Because barnet is an ett-word requiring a neuter adjective, you are forced to swap the adjective for a synonym like slött.

Is slö (the base form of slött) an exact synonym for lat?

It is very close, which makes it the standard workaround. While lat strictly means "lazy," slö physically means "blunt" or "dull" (like a knife). When applied to people, it means lethargic, sluggish, or idle.

If I absolutely must use the word lat to describe an ett-word like barn, how can I rephrase the sentence?

You can get around the missing neuter form by adding an en-word for the adjective to attach to. For example, you could say Barnet är en lat person (The child is a lazy person) or Barnet är en lat unge (The child is a lazy kid). The plural form also works perfectly, as it is not defective: Barnen är lata (The children are lazy).

Does medan function exactly like "while" in English here?

Yes! Just like "while" in English, medan can indicate both time (simultaneous actions) and contrast (whereas). In this sentence, it is used contrastively to compare the boy and the child.