Keeping a roof over someone's head literally means "providing a house for", but is usually used to express providing for someone in general by giving them a place to live, food, and other needs. Talking about a job using the phrase implies the job either only pays for basic needs (doesn't pay well), or the job is unpleasant, but the person does the job because it pays for the things they need. Because Lorna is mentioned, the sentence seems to mean: "I don't like working for Lorna but the job pays for the things my kids need"

'providing a roof over some person's head' is an expression that means looking after that person by giving them a place to live. Usually it means providing food and other basic comforts as well. So in your example, the speaker is working to provide for his kids. ie. Giving them the essentials for living - a place to live (roof over their head) plus food and other necessities.