I feel like we're parenting our grandchildren and I'm finding that I don't want to do it anymore.

Tell the children's parents as calmly as possible, that you are not able to cope with child-care any longer. You simply feel too old and tired. Give them a month or so to sort something else out.

Your grandchildren are not your responsibility, however much you love them.

The bottom line is that if you can't control the children don't have them at all as it's dangerous for the kids to be uncontrolled. Tell the parents so even if this spoils their holiday.

On the other hand if you can control the children then do so whenever they are in your charge

PS there is no shame in finding children difficult to control . Childcare is a special skill that has to be learned.

I think you need to cut right back after the holiday and make it a maximum of so many hours a week that suits you and you can manage and enjoy . The holiday is booked so you're a bit stuck with that is it term time ? Or can you get them booked into daily activities to lighten the load ?

You pay for the two holidays you go on with them each year! They really are taking the p***. Surely you can, then, afford to take a 'special holiday' for yourselves, preferably with a stretch of ocean between you and the family. I was a young(ish) granny when I was presented with DGD 1, and was too busy (and, by that time, single) to undertake a caring role though I took her and her half brother for outings at weekends when I could. Other GC grew up 150 miles away - there is a lot to be said for distance 'lending enchantment'. Parents paid for nursery care and/or childminders and their careers have not, in any way, suffered. Now, in old age, I have five adult GC with whom I have - dare I say - a more loving relationship than if I'd taken a quasi-parental role.

I've read the responses to the OP with great interest as I am in a similar position. My advice would be to think carefully about what actually makes you happy in life and whether you might regret potentially cutting off the regular contact you currently have with both the grandchildren and your AC. Cutting back on the time when the parents are not at work is one thing but if they genuinely need you then you are going to hurt them if you withdraw and that will have consequences.
I regularly do a day or two on my own at DD's and take total responsibility for 3 GC aged 9, 6 and 3. The youngest has epilepsy and complex needs. DD and I know I won't be able to do this forever but despite the travelling (150 miles round trip), overnight stay every week, plenty of cheek and tantrums from the older two, usual mother/daughter tensions and her partner who struggles with me being there so often but has no choice, I plan to do this for as long as I can as I believe I would be far unhappier if I was to stop.
For me it comes down to this - I value my relationships with my 3 AC very highly and I really can only be as happy as my unhappiest child it seems. This family do need my support and there is great satisfaction in still being very useful and capable. I am involved in the parenting (that I try to keep within the guidelines set by DD). This brings its stresses but I have a very special bond with my GC. The other Grandma (who has become a good friend of mine) only sees them on high days and holidays and doesn't know them in the same way as they almost play a part when she sees them. I know she prefers this - we are all different...
Anyway be careful what you ask for as more free time, holidays and travelling are not always guaranteed to make you happy....Good luck

YANBU at all. I do sympathise though. I had my eldest GS for 3 days a week from 9 months old when my daughter went back to work. And like you, I felt like the parent, not the GP. And was exhausted. In the end I told her I could only do 2 days and he went to nursery for the 3rd day. The upside is he's now 17 and we're still very close but I found it really hard at the time.. and I was 16 years younger than I am now. I also did a lot of overnights. With the others, I still pick 2 up from school 3 days a week and help in the holidays. I find the constant clock watching and restriction on our time , having to plan everything in advance as we're needed for childcare a little annoying. Cooking twice 3 evenings a week, ( The kids eat early) once for the GC and later for us is tiring but its impossible say no as it's a necessity so my daughter can work. No other GP involved in childcare for my daughters kids. Thousands of GP are sadly in the same position and I really don't know what the answer is. It takes the joy out of what should be a beautiful relationship. But before resentment builds, it's best to say how hard you find it now

What on earth would these parents do if their own parents didnt take on so much childcare?

Can they really not afford to pay childminders, because a lot of the people I know are single parents and they pay childcare costs.

Some pay almost all of their wages during school holidays, but they manage.