Learn French: Un garçon et une fille sont dans le parc.

French nouns have a gender (masculine or feminine). Garçon (boy) is masculine, so it uses the masculine word for a or an (un). Fille (girl) is feminine, so it takes the feminine word (une).

Why do we use sont (are) here? What subject pronoun does a mixed group equal?

Sont is the they form of the verb être (to be). In French, if a group has a mix of masculine and feminine people or things, the grammar defaults to masculine plural. So, a boy and a girl equals ils (they, masculine).

If we wanted to emphasize the subjects using a stressed pronoun, which one would we use?

Because a mixed-gender group defaults to the masculine plural, you would use the masculine plural stressed pronoun eux. If you wanted to add emphasis like Them, they are in the park, you would write: Eux, ils sont dans le parc.

Why does the sentence use dans le instead of au for in the park?

Both are correct and very common! Dans le parc specifically means physically inside the park. Au parc is a more general way to say at the park.