I think it's really sad.
From the little you have told us I would assume:
Single Parent
Not well off, having to work full time on a low wage
No local family/support
I'm less concerned about them walking home alone, depends on distance etc, and more concerned they are spending too much time alone and without parenting.
Are there any siblings?
I wouldn't call the SS, how is that going to help if the parent loses their job?
Assuming you are not the parent of the child:
I would try and find out about local clubs - my local church run very cheap holiday clubs, why don't you offer to have the child round after school/in the holidays?
UC pay for a big percentage of childcare costs - if parent is on UC that could help? (Has to be Ofsted reg).
You are part of this child's community, if you want to you could help in so many ways.
Is there an after school club the child could stay at and get dropped home by another parent? (Maybe school could be persuaded to use PTA funds to cover the cost).
Could you organise a rota of fellow class mates asking the child to join them at home after school to play?
There are so many small kind acts you could make and encourage others at the school to make which could resolve this issue so easily.
It takes a village to raise a child.
If this is your child and your circumstances, you need to reach out and get support from the school/a church etc to help you. A child being alone for so much time is so sad.