Previous work in English has found that the spelling difficulties of children with dyslexia affect the overall quality attributed to their written compositions. The aim of this study was to investigate whether different processes related to transcription, translation and ideas proposing/planning are affected in developmental dyslexia and to what extent potential deficits are associated to poor spelling. Compositions handwritten by Spanish-speaking children with and without dyslexia aged 9-12 years-old (n = 42) were compared on measures of productivity, spelling accuracy, legibility, lexical diversity, punctuation, sentence structure and grammar, organisation, ideas quality, and readability. Children with dyslexia performed worse in spelling, lexical diversity, syntax and grammar and ideas quality. Interestingly, in the group with dyslexia spelling accuracy contributed to lexical diversity, while lexical diversity was the only significant predictor of syntax and grammar, organisation and ideas quality. This pattern of results was absent in typically developing children. This evidence suggests that spelling difficulties reduce the lexical diversity of the texts of children with dyslexia, which may affect the activity of the translator and the proposer, diminishing the perceived quality of their written compositions.