Learn Polish: Oboje dzieci chce serwować. - Both children want to serve.

Oboje is a collective numeral. In Polish, you must use collective numerals for mixed-gender groups and for certain nouns, most notably those that refer to young beings like dziecko (child). Because of this rule, "both children" is always oboje dzieci, never oba dzieci.

Why is the verb chce (singular) used instead of chcą (plural) when there are two children?

This is a special grammatical rule for collective numerals. When a collective numeral like oboje, dwoje, or troje is the subject of a sentence, the verb must be in the third-person singular in the present tense. It feels strange in English, but Polish treats the group of children as a single grammatical unit here.

What case is the word dzieci in this sentence?

It is in the genitive plural. When you use a collective numeral as the subject of a sentence (in the nominative case), it forces the noun that follows it into the genitive case.

What kind of "serving" does serwować refer to?

It is a versatile verb. Serwować can mean serving food or drinks (like a waiter does in a restaurant), and it is also the exact word used in sports for serving a ball, such as in tennis or volleyball.