Sorry but I think it's too much of an ask. I'm a teacher and have the holidays, but I don't look after other people's children during this time, as my time for my children, my friends and my social life term time is incredibly limited, so I make up for it in the holidays with lots of days out etc...
Unless the children are the same age / gender as mine, I have found there's lots of fighting with my DC wanting to play with one child, and then one child or another gets left out.
Then there's the fact that the children play and make a mess of the house and seem to create a whole lot more mess. For example, if it's just my two they'll play together (one set of mess), but if it's two children, they'll go off in pairs and leave two sets of mess. And when it's just mine, they know they have to fully clean their rooms, so they tend to get less stuff out, But other people's children tend not to care about this and generally are not very good at tidying up after themselves (unless you really nag them, and I don't want to be doing that during my holidays) so that ends up creating a whole lot more work to deal with.
Finally, I like to have lots of days out, but with 4 that quickly gets expensive. I will often meet up with friends and their children most days of the holidays. Again, bringing in extra children to these friendships can be a pain (esp if the children are not the same age / gender as yours) with children getting left out. And then paying for x4 entrance fees and x4 ice creams racks up. I can't afford to pay for other people's kids, but I save up so we tend to do something most days of the holiday. But just a simple trip to the trampoline park for an hour + lunch + slushie and snack will easily cost £25 per child - and that's not even the whole day. Is the parent going to pay this?? You can get child care cheaper than that at some places.
So no, I wouldn't. I've had people expect it of me in the past, and now I just refuse. I also think a whole week and overnight care is cheeky too. That's asking too much imho.