That sentence is taken from April Fool's day by Pete Humphreys
Here is a larger excerpt from the story:
It was late March and Danny was studying the flowers on the route between his parents’ house and school. Rollo lived next door and because their parents were friends Danny was forced to walk in with him. Rollo was not like Danny at all. That morning
The word "mum" indicates British usage, and I'm in the US. In the US, this would be unusual usage, but in the UK "to walk in with him," in the context of a school morning, apparently means to walk to school with him.