I'm similar to Indigo. They just have to do it. But, during holidays, they do some work on some days, and also get plenty of completely free days from work. During holidays, we will have a leisurely breakfast, and then get down to work after that. I have 3 dcs, so all together, at the kitchen table, in the morning, works best. We get it out of the way, each dc follows their own particular schedule and I'm quite regimented about it. But when it's done, that's it, and they are free to do whatever, and we are thereafter quite anarchic. I aim for about an hour and a half 3 x a week during holidays for the older ones, which we feel is a good balance for the work they need to do, and allows enough 'down time' for the rest of the day and leisure days for the rest of the week. I do not offer rewards, sanctions or anything else - they just get into a routine. I think you just have to mean business, OP, and find a time during the day which suits you best, and stick to it. Work out how much you think they need to do, depending on their ages, and go from there.
Our morning 'work' is always of a written nature (whether they are doing maths on the computer or some English work etc). Reading and any music practice etc is always done in the evening, as a more relaxing activity, and is not counted as the 'work' we do in the morning, i.e. that goes on for practically the whole holiday period, unless we are away. Works best for us.
Try to recognise your pressure points regarding homework etc, and if you start to feel yourself getting stressed, walk away. It's counter productive for all. Just remain calm, set your expectations, and be prepared to start again with a fresh slate again the next day, if need be. You won't have to do that too many times for the message to get through.
I think some sort of structure like this, especially during the long summer holidays, is vital, personally. But, it's all about balance.