Granite,
Children at an early age experience grief also.
Would suggest that Winstons Wish is contacted (website has already been posted for this by LeahE are very helpful).
Would also mention The Childhood Bereavement Network: 020-7843 6309; www.childhoodbereavementnetwork.org.uk
Would suggest also that a memory box is made up for this child's mummy. Put in everything you can think of relating to her; tickets to concerts, passport, photos, an item of clothing, letters with her handwriting on it, her scent. condolence cards. Infact anything relating to her, get her friend's memories of her and record them.
Talk to her as much as possible about her Mum. Use the word "died" rather than "sleeping" or "poorly" (as this can instill fear of sleep or being unwell in a child).
In the Sunday Times website today there is an exceelent article in there today about people experiencing bereavement of a parent when they were a child.
I will see if I can post a link.