Learn Polish: Małe dziecko nie ogląda serialu. - The small child is not watching a TV series.

This is the Genitive of negation in action! The verb oglądać (to watch) normally takes the Accusative case for its direct object. However, when we add nie (not) to make it negative, the direct object must switch to the Genitive case. So, serial becomes serialu.

Serial is a masculine noun. Shouldn't its Genitive ending be -a?

Masculine inanimate nouns (objects, not people or animals) are a bit tricky in the Genitive case because they can end in either -a or -u. There is no perfect rule, but loanwords (words borrowed from other languages, like serial) very often take the -u ending.

Why is the adjective małe and not mały or mała?

The adjective must match the gender of the noun it describes. The word dziecko (child) ends in -o, which means it is a neuter noun. Therefore, the base adjective mały (small) must take the neuter ending -e, becoming małe.

The translation says 'is not watching'. Can this also mean 'does not watch'?

Yes! Polish doesn't have a separate continuous tense like English (is watching vs. watches). The present tense verb ogląda covers both meanings. You rely on the context to know if the child isn't watching a series right now, or if they just don't watch series in general.