Olivia,
I think you're right to want to give hsing a little more time. The first year is usually the hardest for most families, especially if you're making the transition out of public school. Our family hasn't had that experience, and I can see how it might be frustrating for both mom and students.
My first question is how old are your children? If they aren't preschool or kindergarten age, I don't see any reason why they shouldn't respect your need for computer time while they're working on school. I would suggest that you give them a challenge such as: "I'm going to set the timer for 15 minutes, and I need you to work on your math lessons. Let's see if you can finish this page before the timer goes off. If you can, you'll get 15 minutes of free time before we go on to the next subject. If you get up and run off to play while the timer is still going, then I will set the timer again and again until you're able to focus and get this lesson finished." Of course, if your children are older, that might be a bit of an elementary approach; then again, if they're older, they should be able to do as they're told. (Please forgive me if I'm being a bit forward here. 
)
My second question: Have you read Laying Down The Rails? It is a very helpful resource on habit training--something our family was definitely lacking in. It doesn't just teach obvious habits like personal hygiene or neatness; it also talks extensively about the habits of obedience, truthfulness, and attention--the 3 most important!
My third question is are you sure the morning is the only time you can have school? I know most people would prefer to have school in the morning when the children and mom are fresh and ready to start the day. However, if you must leave the children unsupervised while you work on the computer, perhaps they could work on their reading until you're finished with your work.
If you're committed to the homeschooling process, make sure your husband knows that. When we first started homeschooling, my husband asked if I planned on doing it until the children were finished with school. I told him I was. Ever since, he has been a huge source of encouragement because he knows it is the best choice for our family. If I ever have those days when I'm just dying to send them off to public school (and believe me, I have those days!), he always tells me what a great job I'm doing and that we are committed to this process together, despite the hardships. If you and your husband have agreed that hsing is the best choice for your family, ask him to encourage you, especially on days like today. He might also be able to talk to the children about their refusals to get their work done while you're working.
I'll leave you with a quote from Charlotte herself: "There is no need to rate the child, or threaten him, or use any manner of violence, because the parent is invested with authority which the child intuitively recognises. It is enough to say, 'Do this,' in a quiet, authoritative tone, and expect it to be done. The mother often loses her hold over her children because they detect in the tone of her voice that she does not expect them to obey her behests; she does not think enough of her position; has not sufficient confidence in her own authority" (Vol. 1, p. 162).
I hope at least some of this is helpful.
~Lindsey
But at first it needs practice as everything -- makes perfect.