Translation:The child was sitting in a corner and he was eating a Christmas pie.
chab covers a large class of baked, bread-based goods. Donuts, pie, croissants, pizza, and dumplings are all chab. I don't think we've ever seen that it covers pudding.
As for exactly what a QISmaS chab is supposed to be, the sentence doesn't say. It could be anything. It could be a Christmas pizza. We don't know.
In the UK the word "pudding" is used like the word "dessert" in the US. Thus many types of "pudding" would be described with the Klingon word chab, though the custard-like dessert known as "pudding" in the US would not be chab. And a "Christmas pudding" is a specific kind of chab: https://en.m.wikipedia.org/wiki/Christmas_pudding
Which is part of why it is not translated as "Christmas pudding" in this exercise (that plus the original poem says, "Christmas pie"). I was commenting on the fact that when E.T.Gregor said "Christmas pudding", he did not mean pudding like we mean in the US and he was most likely thinking of something that would indeed be translated by the Klingon word chab. The course is following American English, E.T.Gregor was not, so I was clarifying.
I'm sure this sentence was inspired by Little Jack Horner. It is not clear, in the poem, what kind of "pie" his "Christmas pie" is, though it is said to actually refer to politicians lining their pockets with public funds. I assume it is a Christmas pudding since Little Jack Horner pulls out a plum.
Accidentally reported this one, except there was a small error... I attempted to translate it in the present tense (which you can't do since the token for "is" is missing) so I just left it out... it accepted my answer, but then said I was missing "is" which is not possible since there is no token for it... However, that all being said, I can see where you can make the also correct past tense translation with the available tokens, so this is probably moot anyway... So, very long story short, disregard the report...