Learn Arabic (Modern Standard): ذهب الجميع إلى الزلاقة إلا طفلا واحدا. - Everyone went to the slide except one child.

In Arabic, when you have a positive, complete sentence and use إلا (meaning "except"), the noun that comes right after it must take the accusative case. That is why we see the double fatha ending on طفلا and its matching adjective واحدا.

Since الجميع means "everyone", why is the verb ذهب in the singular form?

Even though الجميع refers to a group of people in meaning, it is grammatically a singular masculine noun. Therefore, it takes the standard singular masculine verb ذهب rather than a plural verb.

Why is the word for "one" (واحدا) placed after the word for "child" (طفلا)?

Unlike higher numbers, the numbers 1 and 2 act exactly like regular adjectives in Arabic. They must come after the noun they describe and match it completely. Since طفلا is an indefinite, masculine, accusative noun, the word for "one" takes the exact same properties to become واحدا.

Why does طفلا have an extra alif at the end of the word?

The extra alif is a spelling requirement added to carry the "an" sound (tanween fatha) of the accusative case marker. You will see this added to most indefinite nouns in the accusative case, except those ending in a taa marbootah or a hamza preceded by an alif.