I doubt if there are any rules specifically for pools, or if there are I've not heard of them. I expect it's a case of you having to use your judgement and do appropriate risk assessments/get parental consents.
I don't think that a pool like that is inherently unsafe, it really depends on the person in charge. As long as the CM never, ever leaves the children unsupervised it's fine. But you aren't supposed to leave children unsupervised at all when you're childminding anyway.
I'd been thinking of putting my paddling pool up tomorrow when I'll have a 2 year old mindee as well as my own 3 kids. It's a lot smaller than the one your childminder has and I intend to bring everything outside that I'll need to ensure there is absolutely no 'popping inside'. If I do go in for anything then both my own and the other 2 year old come with me. And I WILL stick to that so the pool will definitely pose no danger to the children.
Can you trust your CM to be that careful? If not then you are quite within your rights to simply say that you don't want the children out in the garden while the pool is up. End of. I would respect that as a CM.