Why not try this idea http://hltastaffroom.blogspot.com/2012/01/letters-and-sounds-apply.html I use this with my SEN group. Children correcting their own mistakes is very powerful. Another idea is to write lots of sentences at their level (so they can read) and make deliberate mistakes. Leave off a full stop, start with a small letter, put a question mark instead of a full stop etc. Have them on strips so the children have to sort them. They could then write the correct version in their books. Children only get a sense of what a sentence is by reading out loud to someone. So practising reading out their writing to you would help. You could get them to come up with sentences verbally. Show a picture and ask them to come up with a sentence or give them a word and ask for as many sentences as possible. Practising saying sentences before writing them will help them understand.