It's only a subtle mention that comes as one of Ralph's extended thought monologues. We know already that Ralph's father is in the navy: and that information plays out in the passage.
Once, following his father from Chatham to Devonport, they had lived in a cottage on the edge of the moors. In the succession of houses that Ralph had known, this one stood out with particular clarity because after that house he had been sent away to school. Mummy had still been with them and Daddy had come home every day. Wild ponies came to the stone wall at the bottom of the garden, and it had snowed.
Daddy clearly does not come home every day now - there's a war on, and he's fighting in the navy. But Mummy is no longer with them. Has she left? Is she dead? We don't know. But it is the one time that the word 'Mummy' appears in Golding's novel. So there clearly is some issue here. But the passage goes on to make the point, not that Ralph's life was difficult, but, actually, that 'everything was all right':
And the books—they stood on the shelf by the bed, leaning together with always two or three laid flat on top because he had not bothered to put them back properly. They were dog-eared and scratched. There was the bright, shining one about Topsy and Mopsy that he never read because it was about two girls; there was the one about the magician which you read with a kind of tied-down terror, skipping page twenty-seven with the awful picture of the spider; there was a book about people who had dug things up, Egyptian things; there was The Boy’s Book of Trains, The Boy’s Book of Ships. Vividly they came before him; he could have reached up and touched them, could feel the weight and slow slide with which The Mammoth Book for Boys would come out and slither down... Everything was all right; everything was good-humored and friendly.