Learn Finnish: Kumpikin pentu on todella suloinen.

In Finnish, kumpikin behaves grammatically like "each of the two". Because it points to each one individually, the noun (pentu), the verb (on), and the adjective (suloinen) must all be in the singular form.

Could I use molemmat instead of kumpikin in this sentence?

Yes, but the grammar would change. Molemmat (both) groups them together and takes plural words, so you would say Molemmat pennut ovat suloisia. Kumpikin treats them individually, which is why everything stays singular.

How is the word kumpikin built?

It is made of the question word kumpi (which of the two) plus the suffix -kin (also). When inflecting it for other cases in future lessons, remember that the kumpi part changes but the -kin always stays at the very end.

Why use todella instead of tosi for "really"?

Both words mean "really" and work perfectly here. Tosi suloinen is very common in everyday speech, while todella is a standard adverb form (literally "with truth") that sounds slightly more formal or emphatic.