I'll explain each part. Maybe it is easier to understand this way: "The ceiling of her nursery was blue, with starts in it..." The ceiling of the baby room was blue. It also had stars painted on it. "...as like the sky as they could make it." They (the painters) tried to make the ceiling look real ("as like the sky"). They tried their best ("as they could make it") - note, when stars are in the sky, it is not blue. So they tried to make it look real, even though it is not real. "But I doubt if ever she saw the real sky with the stars in it..." I don't think the baby (she) has ever seen the sky. "...for a reason which I had better mention at once." Why did the baby never see the sky? I should tell you right now. - I would expect the baby is sick, so it cannot go outside. Or maybe the baby is blind.

아기 방을 하늘이랑 제일 비씃하게 그려났어요. 그치만 실제로 하늘을 못볼거같에요 그 이유는 말을 아놔는게 낮다생각 합니다. 아마 계속 읽어보면 그이유가 뭔지 나올거깉에요. Sorry if my Korean is bad I've been studying for only a short time now!

The second sentence means that, although the ceiling of the nursery was painted to look like the sky, she (the baby?) couldn't have known whether the painting was realistic or not. Right after the second sentence, I expect the narrator to give the reason.

Cel in her (children) room was like a sky with the stars, and author guessing if she saw real stars in her life)) About reasons- the authore wrote smth what may help readers understand why the author doupting if she saw a real stars before..