The answer is...
Mis primos creen en Santa Claus.
Hope it helps!!
Well, it does seem correct - the tenses are used correctly, the articles agree with the pronouns/nouns in the sentence, so I think it is good.
There is only one thing, but I don't know if that is a typo or what - there should be a period between limpio and <span>él because those are two separate sentences. </span>
The singular form of los libros is el libro. The reason why we use el is because libro is masculine.