Learn Romanian: Copiii sunt șateni, dar mama lor este blondă. - The children are brown-haired, but their mother is blond.

The base word is copil (child). To make it plural, it becomes copii (children). To say "the children", you add the masculine plural definite article, which is another -i. So, it becomes copiii.

Can I use șateni to describe other brown things, like clothes or furniture?

No, șaten (masculine singular) or șateni (masculine plural) is used specifically for brown hair. If you want to say a dog, shoe, or table is brown, you use the word maro.

Why is the word order mama lor instead of lor mama for "their mother"?

In Romanian, possessive words like lor (their) usually come after the noun. Also, the noun itself usually needs a definite article ("the"). So mamă (mother) becomes mama (the mother), and mama lor literally translates to "the mother of them."

Why do the two adjectives end differently (șateni vs. blondă)?

Adjectives must match the gender and number of the noun they describe. Șateni is masculine plural to match copiii (the children). Blondă is feminine singular to match mama (the mother). The masculine singular form is blond.