I'm not a teacher but I've put both of mine through 7+. I just got them to do 20 minutes writing most days. Just try to think of a fun topic like 'A Day in the Life of (pet's name)' or 'If I Could Fly' or 'If I were Invisible'. Set a realistic target for length and don't be too pressured about SPaG. Try to keep it fun.
Not in reception. Probably year 1 after Christmas. Schools round here are looking for potential, rather than ability. So don't get too hung up on detail. Personality is probably as important as academics. Just keep reading with and to your DS. Good luck!