Learn Croatian: Djeca trče prema moru. - The children are running towards the sea.

Because of the preposition prema (towards), which requires the dative case to show direction. More is a neuter noun, and in the dative case, its ending changes from -e to -u, making it moru.

The word djeca ends in '-a' like a singular feminine noun. Why is the verb trče in the plural form?

Djeca is a special collective noun meaning 'children'. Even though it has a singular feminine ending, it carries a plural meaning. When it acts as the subject, the verb must be plural, which is why we use trče (they run) instead of the singular trči.

Since this lesson also teaches k and ka, could I say 'Djeca trče k moru'?

You could, but prema is much more natural here. Prema is generally used for physical direction towards places, landscapes, or objects (like the sea). The preposition k (or ka) is usually saved for moving towards a person or a specific living thing.

Does trče strictly mean 'are running', or can it also mean 'run'?

It can mean both! Croatian only has one present tense. Trče translates to 'run' or 'are running' depending on the context. In this sentence, the continuous 'are running' is simply the most natural English translation for kids heading towards the water right now.