I'd suggest having a word with someone at the school, I work for local authority and often am involved in bringing multi agencies together to discuss an action plan for the family. For example, in this case the school would arrange a meeting where the mother and child's gp would be invited, appropriate representation from social services (children looked after team and assertive outreach if required), a rep from ParentTalk or a similar counselling service, family support, Sure Start, youth service, Housing, etc.
This should then provide the mum with appropriate support and will allow the authorities to monitor the situation. Care / support workers can help the family get back on their feet and offer further help if desired (counselling for bereavement and / or alcoholism for example).
Don't feel like you are being a nosey parker, you have to think of what is best for the little ones. It's such a hard position to be in Goods, I know exactly how you feel X