Learn Finnish: Onko tämä lapsi kuudes vai kahdeksas?

The regular number for six is kuusi, and eight is kahdeksan. To turn them into ordinals ("sixth" and "eighth" in the basic case), they change to kuudes and kahdeksas. Notice that many Finnish ordinal numbers end in -s in their dictionary form.

Why does this sentence use vai for "or" instead of tai?

In Finnish, vai is used specifically in questions where you ask the listener to choose between two different options (Is it A or B?). Because this is a direct question asking to pick between "sixth" and "eighth", we must use vai. The word tai is mostly used in regular statements.

Why is onko at the very beginning of the sentence?

To turn a statement into a question in Finnish, you add the question particle -ko (or -kö) to the verb and move it to the front. The verb here is on (is), so it becomes onko (is it?) and starts the sentence.

Why are the ordinal numbers in the basic nominative case here?

In Finnish, adjectives and numbers must match the case of the noun they describe or refer to. Because tämä lapsi (this child) is the subject of the sentence and is in the basic dictionary form (the nominative case), the ordinal numbers kuudes and kahdeksas must also be in the nominative case.