I was a volunteer helper in Early Intervention in our local area, for four years. Families were referred through school, SS, police or whatever. Volunteers were trained and monitored closely, and offered help to parents or children who were experiencing difficulties.
I worked for a few hours at a weekend, taking a child to the park, beach, woods, etc and just provided the sort of experiences and interaction that would, ideally, come from parents, but when parents couldn't cope.
One volunteer supported a family, and helped prevent four children being taken into care, which was a brilliant result. We would sometimes present information at case conferences. Some families feel more comfortable with a volunteer, than with SS.
We also supported at lunch times in some primary schools, with 'drop in' sessions in a dedicated room, so children could just come in and chat if they had 'problems' in the playground, or we provided craft activities.
[With the cut-backs I don't if the service is still operating. I'll try and find out.]