Learn Polish: Dzieci siedzą z przodu. - The children sit at the front.

The word dziecko (child) is highly irregular. Although it is a neuter noun in the singular, its plural dzieci takes a completely different form and belongs to the non-masculine-personal gender category, rather than using standard neuter plural endings. It is a common exception you just have to memorize.

What is the difference between using siedzą and siadają here?

Siedzą comes from siedzieć, which describes the continuous state of being seated (meaning they are currently sitting there). If you used siadają (from siadać), it would describe the physical action or motion of taking a seat (meaning they are in the process of sitting down).

Why does the sentence use z przodu for "at the front"? Doesn't z mean "from"?

Yes, z means "from" when followed by the genitive case, as it is here with przodu (from przód). In Polish, expressing positions relative to the front, back, or sides of something idiomatically uses "from". So "at the front" is literally "from the front" (z przodu). You will see the exact same pattern in z tyłu (at the back) and z boku (on the side).