Her reaction is quite understandable considering the profile of the character. Laura is very young, sensitive by nature, and has always lived in a protective world. The neighbour's accident at work is probably Laura's first real encounter and confrontation with tragedy. Quite naturally, she not only sympathizes but empathizes with the grieving family. To let the garden party go on - oblivious to their pain - seems to be a demonstration of gross disrespect.
Laura learns through the reaction of her family, particulary that of her mother, that life goes on despite its trials and that the world cannot stop dead in its tracks each time someone faces disaster. Sorrow is an inherent part of existence, and its is not disgraceful to sympathize with someone's dilemma without actually suffering for it as well.
When I first read this story (young like Laura), I was a bit shocked by the inured tone of the author. Life has taught me since that she is more right than wrong.