How old are they, so how long do they have before 11+ ?
For English, reading as much and as widely as possible, paying careful attention to spelling of words, punctuation, how descriptions are used, may help.
The children should have access to a good dictionary, also a thesaurus, and a basic 'rhyming dictionary' to help with writing poems.
If, as they come across new or unfamiliar words, they write the words in a notebook, so compiling their own personal dictionary, this may help with spelling. When writing stories, use the thesaurus to find more interesting words.
To help with spelling and 'sounding out' this is a very useful book:
An inexpensive and easy to use book, that can encourage children with reading, spelling and writing, and really help them to understand Phonics, is reviewed in the MN Book Reviews section. Just search ‘Phonics’ and my name.
As they (and you?) get older or on to more advanced writing, text books by Lynne Truss and David Crystal explain how to improve one's writing. There are adult books by both authors, but they also do ones aimed at children.
Let me know if there are specific weak areas, and I'll see if I can help further.