Learn Croatian: Neka maloljetnik pokaže pečat.

The word neka combines with the present tense verb pokaže to form a third-person imperative. In Croatian, this is how you express a command or permission for someone else, translating to "Let him/her/them..." in English.

Why use the verb form pokaže instead of pokazuje?

Pokaže comes from the perfective verb pokazati, which means to show something once as a completed action. We use it here because the minor just needs to present the stamp. If we used the imperfective pokazuje, it would mean showing it continuously or repeatedly.

Pečat is the object being shown. Why doesn't its ending change?

You are right that direct objects take the accusative case. However, pečat (stamp or seal) is a masculine inanimate noun, meaning it is a thing, not a person or animal. For masculine inanimate nouns, the accusative form is identical to the nominative (dictionary) form.

How is the word maloljetnik built?

It is a compound word made from malo (few or little) and ljeto (summer or year). By adding the suffix -nik, which refers to a person, it literally means a person of "few years", which is the standard word for a minor.