aideuri nunsarameul mandeulgo isseoyo.

The ending -고 있어요 is the Korean present progressive (continuous) form. It shows that an action is happening right now. You take the verb stem (만들- from 만들다, “to make”) and add -고 있다, then conjugate 있다 politely as 있어요. So 만들고 있어요 = “are making.”

-이/가 is the subject marker, highlighting who is performing the action. Here it simply marks 아이들 (“children”) as the grammatical subject. If you used 은/는 (topic marker) — 아이들은 — you’d be contrasting or emphasizing “the children” as the topic of conversation, but the basic meaning stays the same.

-을/를 is the object marker. It shows what the verb acts on—in this case, 눈사람 (“snowman”) is the direct object of 만들다 (“to make”), so we say 눈사람을 만들고 있어요.

In modern Korean, 눈사람 is written as one word (a compound noun). Writing it as 눈 사람 (two separate words) would be less natural and may confuse the reader, since 눈 alone is “snow” and 사람 is “person.” The compound 눈사람 specifically means “snowman.”

You can simply raise your intonation, or add 까 for a more formal question:
• Intonation only: 아이들이 눈사람을 만들고 있어요?
• Formal: 아이들이 눈사람을 만들고 있습니까?

They’re the same verb form (progressive) but differ in politeness:
• 만들고 있다 – dictionary (plain) form
• 만들고 있어요 – polite present form, used in everyday conversation.

Yes, in very casual or spoken Korean you might hear 아이들(이) 눈사람(을) 만들고 있어 or even 애들이 눈사람 만들고 있어. However, when learning, it’s best to include particles to understand sentence structure.