They don’t take each day as it comes. They assume that you are their default childcare unless something else comes along.
I am very sorry to hear this.
I think I am fairly typical of parents who work full time. School holiday childcare is meticulously planned well in advance, because I cannot afford to casually swan along and then not have a plan in place one day! It would be so incredibly stressful to scrabble around trying to sort things out with just a couple of days to go. Presumably they don’t feel this pressure as they just assume that you will take them.
They need to do what most other people do, which is a mix of each parent taking annual leave by themself for childcare (or unpaid leave) and holiday camps for any periods they are not on family holidays. Holiday camps can be tricky as extended hours aren’t always quite long enough. We manage that by sharing the load with another working family, and between us we can make it work. My husband’s employer is happy for him to shift hours if there are no meetings, so he can start a little late. It then doesn’t matter that he finishes late, because another parent has done the converse, and is there for the pick up. There are also a couple of days each week when an office-based parent can work from home, so they don’t have the commute. There are ways to make it work, but you have to be organised. This is why I cannot believe that your relatives’ plan is to take things day by day, and not simply that their plan is that you are their childcare.
I would NEVER presume that a SAHP would just take on my children as free childcare. I realise that, even though time with your own children is a joy, the summer holidays are long, and it’s stressful to entertain hot, sticky, tired children single handed for 6 weeks. And then to just assume you could easy add another THREE children to the mix on a frequent basis, it really takes the biscuit.
Keep standing firm on this. I know it’s of no practical help, but I am behind you!