Learn Finnish: Nuorin pikkuserkku on lapsellisehko. - The youngest second cousin is somewhat childish.

The suffix -hko or -hkö (depending on vowel harmony) means 'somewhat' or '-ish'. It always attaches to the inflectional stem of the adjective. For words ending in -nen, the stem ends in -se-. Therefore, lapsellinen becomes lapsellise-, and adding -hko gives us lapsellisehko.

Does pikkuserkku mean a cousin who is young or small?

No, pikkuserkku specifically means a 'second cousin' (the child of your parent's cousin). If you wanted to talk about a first cousin who is simply young or small, you would use a regular adjective and write it as two separate words, like pieni serkku or nuori serkku.

Why is the superlative nuorin used here instead of nuorimmainen?

Both can translate to 'the youngest'. Nuorin is the standard superlative form of the adjective nuori (young). Nuorimmainen is a slightly more specific or affectionate word, almost like a noun, that is primarily used for the absolute youngest sibling in a nuclear family. Since this sentence is about a wider family relation, the standard superlative nuorin is the natural choice.

How would you inflect lapsellisehko if it needed to be in a different case, like the partitive?