The comment that struck me OP was when you said they come through the door and “it’s party time”.
This is really the crux of the problem.
They arrive in holiday mode and view your house like a rented AirB&B, complete with catering, maid and baby siting services included.
Which is why you are “uptight” when you are not in the same “zone” as them.
The thing is, it’s not a holiday rental. It’s your home and you are not their personal skivvy whilst also working/parenting your own children.
To a degree you have made a rod for your own back by not pushing back sooner, so it’s well overdue to spell out some home truths and ground rules.
As a start point I think you need to think about what you are prepared to “offer” in respect of these visits and importantly what you are not.
So for example, a place to stay, access to laundry facilities, use of the kitchen etc is all fine.
What you will not be doing is buying food for them for two weeks, baby sitting, running around tidying up after them.
It is perfectly reasonable to say that whilst you want them to visit, you can no longer manage their expectations and that something has to give.
That means them re-setting their perception of what staying with you means - it’s not a holiday let and if that’s what they want then they need to book that for some or all of their visit.
Staying with you means respecting your home and the fact that you are NOT on holiday, nor are you there to facilitate their vacation.
In practical terms that means keeping the house tidy on a day to day basis. Keeping belongings in their rooms and not dumped everywhere. Coats/shoes put away. Washing their bed linen and making beds before they leave.
Buying their own food (and frankly in their position I’d be cooking for you as a thanks for letting me stay, especially if you are working).
Finally, no baby sitting and being respectful around morning/bedtimes especially during the week when people are working.
You need to break the habit they (and you) have fallen into. Staying at someone’s home and being “on holiday” are actually very different propositions and they need to be reminded of that.