Translation:The moon is round, the mooncake is sweet.
I'm not sure it makes sense to say that a sentence associating mooncakes with the moon is random. That's like saying that a sentence associating Christmas cake with Christmas would be random.
月饼, or mooncakes, are eaten on or around 中秋節, which occurs at the time of the autumn full moon, and moon worship is part of the festival (which is of course why they're called mooncakes).
This is a silly rhyme that some people find fun to say at that time of year – presumably mostly children, or adults in entertaining children. (It may be regional.)
I'm not sure how to report this, but for arranging the words in the correct sentence, I was only given one "The" in the word bank so no matter what my answer would be incorrect