To be really struggling with the summer holidays?

Play dates so they each have a friend over - arrange for them both to visit friends on the same day so you get some time off. Aren;t there any free museums near you or parks where there are animals? And forget the housework.

You are being unreasonable on yourself.
Maybe your DH could go to the office or find a work space in your area.

If you have a garden hide things out there and tell them to find certain things. You give them a Picture list of what you want found & put some stuff in the list that isn’t in the garden. So your DC can’t just tick things off.

Give them rules

1. They should not move the item.
2. They can put down any bugs they see.
3. Will you come out once the washing up has been done.

Tomorrow the nine year old can do the washing up of the breakfast things. You will dry up. Then 4 year old can help with stacking up plates & you can put them away. — This gets them used to doing the dishes. Every home has dishes for the foreseeable future.

I recommend planning 7 days of things and rotating. So day one could be supermarket shop and Taco night. Day two, park trip and painting etc. It brings some routine to it.

I also recommend audiobooks and headphones while you get on with things.

And invited play. So just dump a toy they haven't played with for a while in the middle of the sitting room.

Or ask them to sort toys out to go for charity. Guaranteed hour of demonstrating they must have every toy.

Can your DH not go and work elsewhere? Your life would be much easier and cheaper if your kids could enjoy their own home.

Can your husband work somewhere else for a few weeks?

It's normal for children to need to settle in to the holidays, and in my experience, for them to be fed up of each other in the last week - a sign that they are ready to go back!

Need to be able to be at home some of that time though. Not reasonable to expect you to be out all day, every day.

Can you set a timer for 15 minutes and say this is your special mummy playtime and do 15 mins play of their choice, then set up an activity for them for 10 mins while you stick a wash on etc?

My children are generally happier with friends around especially at age 8+ so can you arrange some play dates for your DC?

I think it's perfectly acceptable to let the house stuff go a bit during the day and your husband can pitch in after work. If he doesn't like the idea, let the kids fight outside his office door while you clean the kitchen!