Learn French: Le garçon et la fille sont prêts. - The boy and the girl are ready.

When a group has both masculine (le garçon) and feminine (la fille) nouns, French grammar rules say the whole group is treated as masculine plural. That is why we add an s to the masculine form to make prêts, rather than using the feminine plural prêtes.

Since we added an s to make prêts plural, does it change the pronunciation?

No. In French, the plural s at the end of regular adjectives is silent. You pronounce prêts exactly the same way you pronounce the singular masculine form prêt.

Why is the verb sont used here instead of est?

Sont is the they form of the verb être (to be). Since our subject is two people (the boy and the girl), we treat the subject as they (ils) and use sont to mean are.

Can la fille also mean the daughter?

Yes, fille means both girl and daughter. You have to use the context of the sentence to know which one it is. Because it is paired with le garçon (the boy) here, it clearly means the girl.